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.

Ran unit tests.