The timestamp wasn't originally specified to be switched to the
"in-context" link, so this commit makes sure that it always points to
the "HTML" representation of the annotation, if that link is available.

Prepare the direct_linking feature flag for removal

Show adder when the selection changes

Fix client on pages that define a variable called `global`

Modules which assume the existence of a global variable called "global"
(which exists in Node) are wrapped by Browserify during the build which
normally results in "global" being aliased to "self" or "window" in a
browser environment.

If code on the page into which H is loaded defines a global variable
called "global" however, that gets used instead and this can break such
modules. In the case of https://www.civilsprep.com, "global" is a
reference to a DOM node for example.

This commit fixes the issue by only aliasing "global" (as seen by the
module using it) to either "window" or "self" and not to any existing
variable called "global".

Fixes #2723

This is now deployed and enabled for everyone so we can remove the flag.

Make "/app" endpoints relative to serviceUrl

As reported by Donny Donovan from EJP, requests to "/app*" related
endpoints weren't correctly being constructed relative to the configured
"serviceUrl", which makes the client less portable.

This change removes a leading slash from the path supplied to the URL
constructor, ensuring that if "serviceUrl" is not at the root of a
domain, the "/app*" paths will still be correctly constructed.

Extract functions for generating and manipulating observable
streams which are not text-selection-specific out of `selections.js`
into a separate utility module.

When using touch input to manipulate the selection there are no
mousedown/mouseup events that we can listen to and the
touchstart/touchend events are not triggered when manipulating the
OS-provided selection handles in the browser.

Instead listen for selectionchange events and show the adder in
response.

To avoid showing the adder every time the selection handle moves which
would be distracting, we instead buffer selectionchange events and only
show them after a pause.

When the user is using mouse input to make a selection, we ignore
selectionchange events so that the adder does not appear until the user
finishes making their selection.

In preparation for some more complex handling of document events in
order to show the adder when the selection changes on mobile, refactor
the processing of events to use observables.

The zen-observable package is used as a lightweight implementation of
the proposed ES2017
[Observable](https://github.com/zenparsing/es-observable) API.

devserver: Extend ALLOWED_ORIGINS instead of overriding it

Add padding, fix hover state colours, tweak shadow

Making this change to increase the visibility of the adder.

Legacy API cleanup

Restructure the documentation

If a selected annotation is unavailable to a user, prompt user to sign in
if they are not signed in. If a selected annotation is unavailable to a user
and they are already signed in, display a message.

See: https://trello.com/c/scNGUMBk/330-improve-copy-to-clarify-direct-linking-functionality

Use human friendly pylint names

Add enter/exit animation when adder is shown

Remove legacy ES code

Usage of this was removed by ba02aee28

The previous logic contained several errors and omissions which could
result in the card not appearing at the top of the viewport after
scrolling.

 1. If the set of visible threads changed multiple times before the timeout
    that triggered re-measuring of visible thread heights changed, then
    a measured height could be replaced with 0.

    Add checks to catch this.

 2. Account for the target scroll offset changing as a result of
    the document height increasing after the actual heights of
    cards become known and consequently the maximum possible
    scroll offset changes.

 3. Correct calculation of the position that we need to scroll the
    window to in order to have a card positioned at the top
    of the window. For the first card in the list, the position
    should be 0 (as returned by visibleThreads.yOffsetOf(...)),
    and for the Nth card it should be the total height of the N-1
    previous cards.

The previous delay made the adder feel laggy. A shorter delay gives a
snappier feel.

Add an entry animation for the adder which makes it 'pop up' or 'pop
down', depending on which direction the arrow is pointing.

Additionally, animate the color transitions when hovering over the
toolbar buttons to soften the changes.

This was missed in 6c0e21a5df.

Fix several missing 'var' declarations in streamer.js

New threading 4/N - Virtualize the thread list

Re-implement the behavior to scroll newly created annotations into view
in the sidebar so that the user can start editing them immediately.

A complication with doing this given that the annotation list is
virtualized is that if we want to scroll to the Nth visible annotation,
we may not know the actual heights of the N-1 annotations above it.

As a result of scrolling the list, some of the heights of those N-1
annotations may be updated from an estimate to an actual measured
height, changing the Y offset that we want to scroll to in order
to make the Nth annotation visible.

There are a few strategies that we could use for dealing with this:

 1. Forcibly render all N-1 annotations and measure their height.
    This is prohibitively expensive for long annotation lists.

 2. Use a cheaper method to calculate the exact height of an annotation.
    Unfortunately we don't have a way to do this at the moment.

 3. Use an iterative approach:

    a. Estimate the target position based on current known/guessed
       heights.

    b. Scroll to the estimated position, which will trigger re-rendering
       and possibly alter the known thread heights.

    c. Re-estimate the target position and if it changed, go to step b.

This commit implements the third method.

These elements are used purely to reserve space for offscreen threads
which are not actually present in the DOM, so they should not occupy any
space beyond that assigned to their 'height' inline style properties.

Annotation cards have a small (~8px) margin at the bottom which needs to
be taken into account when computing the height of each thread in order
to accurately size the spacer element which reserves space for
off-screen threads.

Expand the class description to better explain how virtualization
improves UI performance.

Prior to this unsaved changes were lost when an annotation being edited
was scrolled off-screen and the annotation component was destroyed.

There was already a mechanism in place for persisting unsaved changes
to a temporary store when switching accounts and when switching groups
but this was only triggered when the signed in user changed or the
focused group changed.

This commit unifies the handling of all three cases by listening
for the '$destroy' event which is fired whenever an <annotation>
is about to be destroyed.

Use transform3d() to force the top bar onto a new compositor layer so
that it does not judder around when scrolling the window.

The better fix here will be to make the body scroll rather than the
whole window.

Make the sidebar handle "large" (ie. 50+) numbers of annotation threads
much better by virtualizing the thread list. Virtualizing means that we
only instantiate <annotation-thread> components for threads which are
either in or 'near' (above/below) the viewport. For the remaining
offscreen threads space is reserved by a couple of <li> elements above
and below the visible threads.

Following the introduction of the <annotation-thread> component this is
now unused.

This also removes the JWZ threading implementation since the
functionality this provided is now implemented by `build-thread.js`

New threading 3/N - Render annotation threads using new threading infrastructure

Make the behavior of the new threading implementation consistent with that of
the previous threading implementation by displaying the total number of visible
annotations (+ replies) as the search result count.

Fix "Set changed size ..." error in websocket server