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.

Make Intl an optional argument in time.js so that tests can pass in a
mock Intl (or null) instead of having to patch window.Intl.

Clearer than using an array of arrays.

Clarify why we don't just call Date.toLocaleDateString().

getBreakpoint() always return a breakpoint.

If an annotation is more than one day old then it gets a timestamp like
"09 May". The next time this timestamp would change would be on
1 Jan 2017 when it would change to "09 May 2016". Rather than setting a
really long timer, just never live update "09 May"-format timestamps.

The case where Hypothesis is left open without reloading a page until
the end of the year, and the case where Hypothesis is opened close to
midnight on Dec 31st, are obscure enough to ignore.

If an annotation is not from the current year then it gets a timestamp
like "14 March 2015". The format of this timestamp will always be the
same, so don't set any timeout to update it.

toLocaleDateString() creates a new formatter on each call which is
expensive.

These are used when the datetime in question is more than 24 hours ago
but still in the current year, and when it's not in the current year,
respectively. They replace a few different datestring formats that were
previously being used for datetimes in this range.

This produces "1 days ago" which is wrong, will be fixed later.

In date strings on annotation cards.

In date string formatting functions. Will enable more diverse
formatting.

In time.js replace format strings like '{} minutes ago' with functions
like nMinutesAgo(delta) that return strings.

The format strings only support datetime string formats of the type "n
seconds/minutes/hours/days/months/years ago". Although we don't take
advantage of it yet with format functions we can format datetime strings
any way we want, for example "16 April" for a date more than 24 hours
ago or "16 Apr 2016" for a date more than a year ago.

Although we're not taking advantage of it yet, this should allow us to
use more complex logic such as "within the current year" to select a
date format.

This is the first part of the new threading UI implementation
whose design is outlined at https://github.com/hypothesis/h/pull/3176/

build-thread implements the logic for constructing an immutable view
model for a list of conversations (a 'Thread') from a list of
annotations and replies plus an object describing the current UI state.

This view model is then rendered by an <annotation-thread> component.

Once the other parts of the new implementation are ready, this will
replace 'threading', 'thread-filter' and the JWZ script.

Make nipsa work with Postgres feature flag

Sync PG annotations to ES

Using custom element names for the adder's container is a simple trick
to reduce the likelihood of the adder's styling being affected by CSS
selectors in the containing page, such as example.com's `div` selector.

Remove compass-mixins dependency

Build Firefox WebExtensions during Travis builds

This is a global available in the extension and content scripts.

 * Use wildcard origin when sending messages to or from app.html
   served from a background page.

 * In embed.js, access the 'chrome' global via 'chrome' rather than
   window.chrome. In Firefox the 'chrome' global is not attached to
   the `window` object.

 * Work-around difference in how content script results are reported
   in Firefox (an object) vs Chrome (an array of objects, one per
   frame).

 * Avoid trying to call APIs which do not exist in Firefox.

Publish events in response callback

The immediate problem this dependency causes is that it it fails to
build with the current version of node-sass (see
https://github.com/Igosuki/compass-mixins/issues/84) which in turn means
that H cannot be built with Node 6.0. Removing this dependency allows
the node-sass dependency to be unpinned so that the current version
can be used.

The primary functionality that compass-mixins was used for was vendor
prefixing, which is now taken care of by autoprefixer, plus CSS fixes
for browsers which we no longer support (IE < 10).

For the remaining handful of mixins for resets and color mixing which we
are still using, this commit just extracts the few lines of code for
each from the compass documentation and includes it directly in our SCSS
files.