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

Postgres to ElasticSearch reindexer

As noted in https://github.com/hypothesis/h/pull/3284#issuecomment-219031557
threads which have been explicitly collapsed need to have their
expansion state reset when the filter query changes.

This commit resolves this by moving the search query and sort UI state
into the annotationUI Redux store and handles the state update
in a SET_FILTER_QUERY action.

This means that the rootThread module no longer maintains any primary
state of its own, just state derived from the current annotationUI
state.

Given a thread like this:

    A-
     |- B
        |- C
        |- D
     |- E

Where only 'B' matches the search filter, the rendered output should be:

    View 4 more in conversation
     |- B
        View two more in conversation

Only annotations, not replies can be selected, so only filter top-level
annotations against the selection map.

This resulted in an issue where 'View N more in conversation' being
shown where 'N' was the number of replies when selecting an annotation
with replies in the page.

See https://github.com/hypothesis/h/pull/3285#issuecomment-218734287

This replaces the rendering of conversation threads using
the new threading implementation.

 * Add <annotation-thread> component to render Thread objects
   generated by rootThread.

 * Expose the Thread generated by the `rootThread` service on
   the scope so that <annotation-thread> can render it.

 * Remove logic and tests in <annotation> for updating reply
   counts and thread states as these are no longer needed.

This adds a service that listens for changes in the UI state and
the set of loaded annotations and generates a thread structure in
response, which can then be visualized by a view.

This commit also adds a first integration test which wires together
the non-visual parts of the new threading implementation.

 * Add 'annotations' to the UI state in annotationUI and update
   this when annotations are loaded or unloaded.

 * Add rootThread which listens for changes in the UI state and
   generates a new conversation thread structure in response.

 * Add an integration test which tests the wiring of the non-visual
   parts of the new threading implementation.

This adds a service that listens for changes in the UI state and
the set of loaded annotations and generates a thread structure in
response, which can then be visualized by a view.

This commit also adds a first integration test which wires together
the non-visual parts of the new threading implementation.

 * Add 'annotations' to the UI state in annotationUI and update
   this when annotations are loaded or unloaded.

 * Add rootThread which listens for changes in the UI state and
   generates a new conversation thread structure in response.

 * Add an integration test which tests the wiring of the non-visual
   parts of the new threading implementation.

Move hypothesis-celery command to h.cli

Prune old and crufty migrations

Add a `hypothesis migrate` command

New threading 1/N - Begin to centralize UI state in annotationUI

Since putting the annotationUI store into the desired state and
asserting that the state is correct after interactions is quite
straightforward, using the real object rather than a fake seems
appropriate at present.

Give this method a more obvious name and improve a couple of other
documentation comments.

Fix updating annotation extras

Websocket docs fixes

This adds the initial infrastructure using Redux for centralizing the UI
state as an immutable object accessible via `annotationUI.getState()`
which is updated as a result of actions from the UI, network etc.

For background on why we want to do this, see the design overview at
https://github.com/hypothesis/h/pull/3176

Additionally this commit removes a couple of tests that checked for
non-mutation of the selected/focused annotation maps and uses
seamless-immutable instead which provides a better guarantee of this,
but only in debug builds.

* Also remove duplicated variables in css.

Do not test annotation schema

Convert h.script to a package

Views code for updating annotations in postgres

* Add an 'h-tooltip' attribute directive which displays a tooltip above
   the associated element as soon as it is hovered and removes the
   tooltip on mouseout or when the associated element is destroyed.

 * Use the custom tooltip for buttons in the annotation card footer
   and remove the labels.

Move EventQueue into h.api and sandbox event notifies

Remove extra margin set by first and last p in an annotation body.

Sheetaluk/336 reply threading

Change display of datetimes on annotations

* Change the background of bucket bar.
* Change margins, line height, font size, default state colour and hover state colour
  for different elements in the annotation card and the replies thread.

Trello cards:
https://trello.com/c/aXCXxzx2/327-update-the-visual-design-of-the-annotation-cards
https://trello.com/b/o2ungX1n/current-sprint

When dealing with more than one second, minute or hour display secs,
mins or hrs instead of sec, min or hr.

In the case of secs it's always more than 1.

The dates passed in to toFuzzyString() and nextFuzzyUpdate() are
actually date strings and not (as in the tests) Date objects, but the
code expects Date objects, so it crashes in production (but works fine
in the tests).

Fix both the production code and the tests to use date strings.