Rather than continuing to track the message counts in the thread
controller, make a separate deep-count directive that exports a
getter / setter from its controller that can be used to query and
increment counters that bubble aggreggations up to any deep-count
directive above.

Instead of firing a `threadCollapse` event when threads collapse
and canceling it from the annotation directive when the annotation
is being edited, the deep-count directive is used to track the
total number of messages in a thead, the number of matches found
by the thread-filter directive, and the number of annotations that
are being edited.

Remove most of the injectables from the thread controller and handle
more of the DOM-specific bits in the link function. The controller
becomes even simpler, but the directive needs tests now.