We've had a longstanding bug where a review request draft would be updated, but

we would still display old cached data for the draft upon page reloads. While

we rely on the ReviewRequestDraft.last_updated field when generating ETags

for a review request page, this field wasn't being properly updated when a

draft gets updated, so we wouldn't catch that the ETag was stale.
In conjuction with /r/13261, this fixes the caching issue by making sure that

the last_updated field gets updated whenever a field is modified in a draft,

or when an associated file attachment is created/deleted/updated.
This change also updates some file attachment unit tests to take into account

the one extra query that is being performed when saving the last_updated

field on review reuqest drafts.

Tested with the /r/13261 change.

Ran unit tests.
Tested updating a review request draft by updating each of the fields,

  information (bugs, depends on, etc) and updating file attachments, saw

  that last_updated was updated correctly.
Tested updating the change description, saw that it didn't update

last_updated.
Tested manually setting last_updated to a value during saving, saw that

  value being used instead of timezone.now() to make sure that we don't

  overwrite any manually set values.