Fix error in session.js when an /app request fails 

Retry requests to /api endpoint

Do not expect to be able to parse valid session data out of responses
with 5xx status codes - eg. gateway errors, internal server errors.

Convert admin to package

Fix KaTeX font urls

Dispatch markdown editor input changes from $apply() block

The input event handler's callback was not invoked inside an $apply()
block, so the $digest function was never called to propagate the changes
in vm.setEditText() in annotation.js. This happened to work previously
because ngModel's $setViewValue() was called to apply input changes,
which internally uses $scope.$apply().

This commit wraps the input events from the markdown editor with $apply
but debounces them. Since $apply() triggers every single watcher in the
application, running it on every keystroke causes noticeable lag
otherwise.

Fixes #3212

Replace homebrew background workers with Celery

Handle requests to the /api endpoint that fail with network errors in
the same way as failed requests to the /app endpoint by retrying the
request with exponential backoff until it succeeds.

Otherwise, if the request fails then `store.(Annotation|Search)Resource`
are not populated and code that tries to make API requests fails.

When a request to the session endpoint fails with a network
error, avoid trying to parse the response in process()

Tidy up the 'resolve' object to use `// @ngInject` and add a comment to
clarify what it is for.

Update package.json with the version as-formatted by npm

Previously the 'watch-css' task updated H's SCSS files when they changed
but did not reprocess vendor CSS files such as the icomoon font
automatically.

The vendor KaTeX CSS file looks for font URLs in the 'fonts/' directory
relative to the location of katex.min.css.

Rewrite these URLs so that they are fetched from '/assets/fonts' instead
of '/assets/styles/fonts'

Note that the CSS references both WOFF and WOFF 2 format fonts but We
only include the .woff version.

Add Postgres UpdateAnnotationSchema

Markdown editor input refactor and additional tests

The editor is only used from the <annotation> component which never
sets this input.

This simplifies the implementation of the <markdown> component slightly,
and makes its inputs more consistent with other recently written components.

 - Replace use of 'ngModel' with 'text' input property and 'onEditText'
   output event.

 - Add tests for preview state.

 - Add tests for rendering and editing annotations that have no text.

 - Optimize editor initialization by not creating the toolbar's DOM
   elements until the user starts editing the annotation, via using
   ng-if rather than ng-show to hide the toolbar in view mode.

Displaying local file names in the Groups document list

Add deleting of annotations from new search index

Convert h.features to package

Add the skeleton of a functional test suite

Remove routes_mapper fixture and use configurator routes

Re-implement collapsible excerpt overflow state checking

In addition to the content changing, the window resizing and media loading,
another way that the excerpt's overflow state can change is if it or a parent
element's visibility changes.

Since there isn't a generic, performant way to watch the visibility of DOM
elements, the change here adds a scope watch which tests for the presence of
the 'ng-hide' class used by the ng-show and ng-hide directives on parent
elements, since that is the only method of changing visibility that we actually
need to support at the moment.

Excerpts update their overflow state outside of the Angular $digest
cycle. The usual way to deal with this is to wrap the update in a
$scope.$apply() call. However, this triggers a full $digest cycle, which
we want to avoid if many excerpts update their state at the same time -
eg. if we have a large number of annotation cards.

This commit instead explicitly invokes the $digest _only_ in the
affected annotation card and then only if the collapsible state of an
annotation body actually changed.

This makes it possible to write isolated tests for just the overflow
state and content style recalculation aspects of <excerpt> components.

It also simplifies the <excerpt> component tests by making it easier to
reason about when the excerpt's overflow state will have been
recalculated and the directive's state updated accordingly.

rAF is not available in PhantomJS 1.x but is supported by all of our
target browsers (IE >= 10).

Previously the overflowing state of the excerpt was computed implicitly
by the overflowing() method, which was called from several other
controller functions.

There were several problems with this approach:

 1. Since this method calls Element.scrollHeight, it triggered an
    expensive synchronous layout flush.
 2. In certain situations, this could trigger a loop where the
    overflowing flag was continually flipped (see #2960)
 3  It was unpredictable when the overflowing state would be recomputed.

This commit instead opts for a different approach which is more explicit
about when the state is recomputed, based on the assumption that the
<excerpt>'s content size is a function of:

 1. The inputs to the <excerpt>'s content (eg. the text of a quote, the body
    of an annotation)
 2. The sidebar window's size
 3. The dimensions of any embedded media

The content size is then computed and updated as follows:

 1. When the <excerpt> is initially created, an async computation of
    the scrollHeight is scheduled. The calculation unfortunately needs
    to be async because the content may contain Angular directives which
    take several $digest cycles to fully process.

 2. The <excerpt> listens for changes to the content's data (via a
    'contentData' property), media 'load' events and window 'resize'
    events and recomputes the overflow state in response to any of
    these.

There are a couple of other fixes here:

 - Use an optional boolean property for the 'enabled' input,
   rather than a function. The app passed a function to this property
   but the tests passed a boolean.

Fixes #2960

Replace node page router

We don't need all the client-side routing part, we only need to be able
to selectively run certain JavaScript code depending on which site the
user is at the moment.
This implementation is not as advanced as the one from the `page`
package, or what is possible with the `url-pattern` package. But we
don't need any fancy URL matching at the moment, once we do we can
revisit and maybe introduce something like `url-pattern` instead of just
a crude `document.location.pathname === path`.

Add tests for authentication policy

Scroll to annotation when clicking at bottom of collapsed quote

Fix 'Location' sort order for annotations

Use require() for templates

For consistency with how other transforms are specified, specify the
stringify transform in package.json instead of in code in the
create-bundle.js script and the Karma config.

Since components now require() their own templates, it is no longer
necessary to use a preprocessor to load them.

Include Angular component templates as strings in the app bundle using
the stringify Browserify transform and use them from directives using
'template' instead of 'templateUrl'.

This is faster, because it avoids going through the async $http
machinery to load templates, but it is also synchronous, which generally
means fewer surprises due to async template loading and specifically in
the context of the <excerpt> directive will be useful as it will make it
possible for the <excerpt> directive to measure the height of its
content in the link function, because all components inside the
exercept's content will have been fully resolved at that point.