RelationCounterField keeps track of each model instance that contain

references to other models, in order to bump their counter fields, and

it tries to notice when a new instance of a model has claimed the same

ID as an old one (for use in test suites). However, it only worked when

the old instances fell out of scope, and not when they were deleted

(which resets the ID to None and prevents a later lookup from

occurring).
The field now tracks the pre_delete signal for the instance, taking

care to reset state and untrack the instance. It also clears up signal

handlers, ensuring we don't have any stale ones sitting around.
Unit tests have been added for these fixes.

Verified that the new unit tests failed before this fix, and pass

after.
Ran the Review Board test suite (with an upcoming change), which was

failing. It now passes after this.