Add direct linking feature flag

Add a feature flag for direct linking and
use the link provided by the API to a view of
the annotation in context when this flag is enabled.

The Promise import is no longer needed because
Promise and Object.assign() are polyfilled globally
in polyfills.js

Add a utility script to assist with CoffeeScript -> JS conversion

Convert 'annotation-ui' from CoffeeScript to JS

Remove the MathJax math rendering fallback

It accepts a list of CoffeeScript file paths on stdin
and for each <path>.coffee file in the input, it writes
a <path>.js output file containing the converted,
reformatted source.

The result is ES2015 source so will require some
fixups as long as we're using ES5 in the main app
codebase.

The current transformation process involves:

 1. Performing an initial CoffeeScript -> JS
    conversion with 'decaffeinate'

 2. Applying some fixups for common issues, currently
    done with dumb string modifications.

    In future I might look at writing a Babel transformer.

 3. Parsing the source with Babylon to check that it is valid ES2015
    If not, the user is prompted to go and simplify the CoffeeScript
    so that decaffeinate can process it correctly.

 4. Reformatting the source with typescript-formatter, which
    takes care of indentation and high-level formatting.

 5. Running the source through JSCS' fix mode which deals
    with smaller details.

Since this is a very simple module with no Angular
dependencies, I've also de-Angularized the unit test.

Fix installation of URL polyfill with js-polyfills v0.1.16

js-polyfills v0.1.15 used to install the polyfill on
`this.URL`, where `this` was set to module.exports in
the context of a Browserify bundle. Consequently the result
was available as `require('js-polyfills/url').URL`.

As of v0.1.16 however, it installs the polyfill on `self`,
which is the Window object in the context of the browser
and so `require('js-polyfills/url').URL` is undefined.

Fixes #3064

The MathJax fallback was inadvertently broken during recent
refactoring of the <markdown> component and has long been untested.

This removes the fallback for the moment and in doing so
provides a clearer definition of what math we support -
the subset supported by KaTeX.

Looking at our public annotations, there is very little use
of complex math currently.

When we find a clear use case for it, we can revisit supporting
additional math beyond what KaTeX supports.

See https://trello.com/c/9KkjsO6v/66-remove-the-mathjax-fallback
for more background.

Add extra context to sentry logging

Merge pull request #3057 from hypothesis/sheetaluk/280-when-a-user-replies-focus-on-and-expand-that-reply

Sheetaluk/280 when a user replies focus on and expand that reply

added missing new line.

don't attempt toggleCollapsed if no parent available

adding a test and a comment

simplifying assert

Convert {app, app-controller, widget-controller}.coffee to JS

Adds an issue template

 - Extract the logic computing a numeric key for
   sorting annotations by location into a separate
   module to make testing easier and add tests.

In preparation for upcoming changes as part of direct linking,
convert the entry point and top-level controllers for the app
to JS.

 * Use a CommonJS require to bring in the 'vendor/jwz' dependency.

Travis Sentry sourcemaps

Throwing an exception in the context of a Promise
catch handler inside through.obj() just results in the
exception being silently swallowed.

Pass it to the through2 callback
so that Gulp exits with the correct status.

This commit reworks scripts/gulp/upload-to-sentry.js to allow for
uploading to multiple Sentry projects at once. This allow us to build
the assets once, and upload them to multiple Sentry projects within the
same gulp invocation.

Run search results through AnnotationJSONPresenter

Fix JS error checking via Hound

Since enabling JSCS, Hound CI reviews for PRs
which contain JS code changes have been failing
to complete.

ThoughtBot have advised us that the JSCS
checker is in beta and that we should simply disable it.
They noted that ESLint has received a better response
from other testers so we might want to look
at that instead.

 - Use the legacy 'esnext' option only instead of both
   'esnext' and 'esversion' for compatibility with JSHint v2.8.x
   on Hound and developers who have the current stable version
   installed locally.

 - Remove the 'maxErrors' option from the JSCS config, as this
   prevents use of the '-x' option to autofix files which is
   a little annoying.

Fix ubuntu dev install docs

Focus input field after creating a new annotation

The previous method of scrolling a newly created annotation
into view in the sidebar resulted in the input field for
the new annotation losing focus.

Note that only top level annotations have an 'id'
attribute set on the corresponding HTML element.
It might make sense to change this in future although
it would add an extra binding. For now I've just noted this.

When a new annotation card was added, the sidebar tried
to focus it but this failed because the sidebar iframe
itself did not have focus.

When a new annotation is created, first focus the sidebar
in the Annotator host and then let the sidebar focus the
input field.

 * Remove unnecessary use of the $timeout service which
   triggers a root scope digest.

Split Travis build up using matrix

This allows `gulp build` to run without devDependencies installed.

Implement tools to enable Chrome extension sourcemaps

Hound CI is currently using JSHint v2.8.0 which
uses 'es3', 'es5' and 'esnext' config options instead of
the 'esversion' option in v2.9.0 and later.

When the H client is served from a local URL (eg.
chrome-extension://ID/scripts/foo.bundle.js), the original
source URLs and source maps are not accessible
to Sentry.

Enabling source maps for extensions is done in three steps,
the first two of which are implemented in this commit:

 1. Changes to the crash reporting to transform local URLs
    into filenames when reporting exceptions.

    This results in URLs for source files in Sentry
    reports which are independent of where the files
    are served from
    (eg. chrome-extension://ID, moz-extension://ID...)

 2. A Gulp task that uploads JS bundles and their associated
    sourcemaps to Sentry using the Sentry API.

    The assets are uploaded to Sentry using just the filename as
    the URL, so the source files and source maps are correctly
    matched up with the transformed URLs produced by the changes
    in (1).

 3. Configuring CI builds to run the Gulp task from step (2).

This commit makes use of ES2015 template strings in
upload-to-sentry.js, so the JSHint file has been adjusted
accordingly.

Remove undocumented /api/annotations endpoint

Convert fuzzy timestamp formatting functions to JS

In preparation for changing the formatting of breakpoints,
convert this module to a plain CJS module in JS.

Clean up application config/settings