Return the correct user from the last update resource

Review Request #1367 — Created Aug. 30, 2018 and updated

Review Board
A while back we started recording who published review requests in the
`ChangeDescription` so that re-assigned review requests showed proper
history (instead of just showing the new author doing everything).
This patch updates the `ReviewRequestLastUpdateResource` to take this
information into account.

As a part of this change, `ReviewRequest.get_last_activity` has been
deprecated in favour of a new method,
`ReviewRequest.get_last_activity_info`, which returns a dict of
information, including the relevant `ChangeDescription`, which can be
used to determine the author of the update.

Additionally, unit tests have been added to cover the last update
resource's behaviour.

Testing Done:
Ran unit tests.

Bugs closed: 4654

Reviewed at

Description From Last Updated

add a comment on line 574

  1. This is a header (header of what?). And this is what I wrote in the header.

  2. this is my general comment -- at the top. Let's see where this goes. And how does this differ from a header?

  3. add a comment on line 574

  4. oooooh, let's add a footer too. See footer, nice footer.

  1. Ship It!