This commit moves most customisations of the upstream Annotator v1.2.x
into annotator_monkey.coffee, which monkeypatches the Annotator object.
Customisations required by the anchoring subsystem are moved into the
"enhanced anchoring" plugin.

With this change, we should be able to replace our inlined Annotator
source with the built JavaScript for upstream Annotator!

This file provides an abstract base class that's only used in one place,
so this commit just moves it into the one place where it's used.

Most of these changes are either syntactic or should not change
behaviour.

This contains only syntactic changes, but it makes it easier to see an
ever-diminishing diff with upstream Annotator.

We monkeypatch BrowserRange as needed in range_monkey.coffee

This is not in upstream and it's not needed any more.

Remove Annotator.Notification.

Move juicy anchoring bits into a plugin of Annotator

Threading events fix and tests

Rather than looping through all threads, use the references property
to find the parent directly. It should be guaranteed to exist since
one cannot reply to something which does not have an id.

Furthermore, when there is no parent reference, the root serves as
the parent. Without considering this, deleting top level annotations
leave stubs in the UI.

Add some tests that the events do what they're supposed to. NB: the
thread method itself should probably be tested.

Import Annotator

Rather than using prebuilt JavaScript in the vendor directory, build the
relevant Annotator plugins from CoffeeScript imported in the previous
commit.

We don't use I18N or provide locale files, so this serves no purpose.

Rather than using a prebuilt vendored Annotator from our fork, this
commit imports the entire forked Annotator source into the h repository.
Our intent is not to maintain this fork within h, but to remove the fork
repository and incrementally move back to dependence on upstream
Annotator.

Introduce Store resource

This makes it explicit that the function is both a constructor and an
angular resource.

Previously, the text between the markup when the math button was pressed in the markdown editor read $$LaTeX or MathML$$. We don't support MathML, so I've changed it to $$Insert LaTeX$$.