Fix bugs in "Depends On" field.

The depends on field had some lingering problems:
* Change descriptions weren't rendered properly, just saying "added" and
  "removed" without actually including the IDs that were changed.
* Creating a new draft wouldn't copy over the old values, which meant that
  making unrelated changes would clear out the field.

Testing done:
* Added and removed a bunch of dependencies.
* Ran unit tests

Fixes bugs 3073, 3139

Reviewed at https://reviews.reviewboard.org/r/5294/