Feature flags cohorts: Add Feature Cohorts admin panel

Add a TokenAuthenticationPolicy for bearer tokens

Remove additional margin at foot of annotation card

Move canonical annotation list sort state to Redux store

This is a step towards getting us to a place where all important state
in the front-end app is managed in the same way - kept in the Redux
store and modified only via actions.

The sort state was duplicated as a property on the $scope and also
stored in the Redux store.

Instead just expose functions on the scope which retrieve this state
directly from the store and dispatch an action to the store when the
user selects a different sort mode.

In addition, the terminology for sorting has been normalised across the
application (sortBy/sortOptions -> sortKey/sortKeysAvailable), and the
ability to sort the stream has been removed, as it was nonsensical and
broken.

Fix icon spacing

There should be 15px, not 20px of margin at the bottom of the annotation
card, just as at the top.

Removing this CSS means that the annotation footer has no bottom margin
and the 15px is provided for all sizes of the card by padding on
.annotation-card.

The annotation action buttons are inline-block elements which means that
whitespace between the tags results in inserted whitespace in the
document when rendered. This rendered whitespace is roughly 4px wide.

Instead, switch the layout of the annotation actions div to use flexbox,
in which no such whitespace is inserted, and add 4px to the margin on
the buttons.

The annotation action icons didn't seem to be equally spaced due to
excess whitespace on either side of the icons within the font glyphs.
This commit updates the edit, reply and share icons to remove excess
whitespace.

Where previously these three icons were 16px wide (and the trash icon
was 9.5px wide) they are now 14px, 13.5px, and 12px wide respectively.

Use the "HTML" link for the annotation timestamp

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.

Fix display of <ul>, <ol> in annotations

URL encode share links

9be804d broke the display of <ul> and <ol> tags in annotation cards by
overriding a bit more than intended -- specifically, it set `list-style:
none;` on <li> in reset.scss when the Compass reset mixins didn't do
that.

This commit fixes the issue by only setting `list-style: none;` on <ul>
and <ol>, which means that the list styles in styled-text.scss can take
effect as originally intended.

The URL query parameter in these links needs to be URL encoded in order
to guarantee that the receiving services can correctly parse the URL.
Sending an incorrectly-encoded URL such as

    http://example.com/hello world.pdf

can result in the parameter just being ignored (by, for example, the
Twitter intent endpoint).

To allow URL encoding strings in Angular templates.

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.