Separate the Host service

.. from AnnotionUISync.

Earlier, the show/hide frame functionality was implemented as a
background functionality of the AnnotationUISync service,
and it was not directly available for the sidebar code.

This commit separated that functionality into a new service,
and exposes it over a public API.

The test have been updated accordingly, too.

Do angular magic differently

This commit changes how we trigger the refreshing
of the markdown previews in the editor.

It has to do with how angular apply / digest cycles
interwene and crash in some cases.

The change was suggested by @RawKStar77 here:

https://github.com/hypothesis/h/pull/2038#discussion_r26867418

Explicitly disallow implicit relative imports

Add tooltips on adder buttons.

Change text of the toolbar button to "New Note."

Require PyJWT>=1.0.0,<2.0.0

Replace flash service with angular-toastr

Add IE autodetect for dom-text-mapper Annotator plugin

JWT Bearer Tokens

Fix #2051

Autocomplete feature for tags

Introduce tagsService which will handle the necessary filtering of tags
and saving them into the local storage in an appropriate format.

s/Heroku/Docker/

Refresh UX

After performing a search, there's already a clear search link in
the view. Without a border around the search bar, the clear icon
looks a little lonely and strange all the way over on the right.
I offer that users have plenty of ways to clear text boxes and
we needn't give them this one.

Requires some small tweaks around various places we have icons in
use. In fact, this actually eliminates a lot of the small tweaks
in favor of more flexible alignment.

Rather than absolute positions and padding and whatnot just set
the width on the input element and set white-space: nowrap to ensure
it and the icon are side-by-side.

Better to use data attributes for signaling the intent than to
be overloading the CSS class.