Refactor annotation controller tests

Remove gloval `controller` variable from AnnotationController tests.

Move the tests for annotation.js's top-level function out of the
'AnnotationController' describe() - these functions aren't part of
AnnotationController and their tests don't need all the setup in this
describe().

Wrap the annotation directive tests in a new top-level describe that's
for the entire file annotation.js, not just AnnotationController or the
annotation directive.

Refactor annotation controller, part 1

Move scope.feature() from the annotation directive to vm.feature() in
the AnnotationController. Just trying to move as much as possible out of
the directive's scope and link function into the controller, if we're
gonna use a directive controller here try to just use it instead of
randomly using the link function and scope for some things and the
controller for others.

It doesn't appear to be possible for model to be null.

Move the DOM event listener function that saves an annotation on
Ctrl+Enter into AnnotationController. This is just for the sake of
keeping all the code in one place (in AnnotationController, not in the
directive's link function) and putting code where it can be tested more
easily.

Note that the controller's save() function was previously called with
scope.$evalAsync() and we're now just calling it synchronously. This
seems to be fine.

Remove AnnotationController's vm.preview as it doesn't appear to be
used.

Replace AnnotationController's vm.editing boolean with a vm.editing()
method.

This removes some duplicated state, as both vm.editing and vm.action
were recording whether or not the annotation was being edited.

Move scope.share() from the annotation directive to vm.share() in
the AnnotationController. Just trying to move as much as possible out of
the directive's scope and link function into the controller, if we're
gonna use a directive controller here try to just use it instead of
randomly using the link function and scope for some things and the
controller for others.

It does not change after directive linking, there's no reason to
$observe() it.

Translate annotation.coffee to JavaScript

Fixes a bug that crept in during the annotation.coffee -> annotation.js
translation.

Declare variables on the same line where they're first defined, instead
of at the top of the function.

This is a translation of commit 363134d28cb7ff8d2313278e1b4b6c09b5bf9bed
from CoffeeScript to JavaScript. Original commit message from
363134d28cb7ff8d2313278e1b4b6c09b5bf9bed:

When saving a draft to the drafts store, don't save changes for fields
which aren't actually set. This prevents us from accidentally later
restoring `null` to one of these fields.

Fixes an issue where creating an annotation when logged-out would result
in a null permissions field.

These were lost when translating annotation-test.coffee ->
annotation-test.js using the `coffee` command.

Tidy up the CoffeeScript-generated code in annotation-test.js.