The ChangeDescription model now supports recording a user that is

associated with the change. The user should be retrieved via the

ChangeDescription.get_user method, which will delegate determining the

user for the change description to the model the change is associated

with if it has not yet been determined. Once determined, it will be

saved to the database for faster retrieval in the future.
The user field can currently only be calculated for change

descriptions associated with review requests.

Ran unit tests.
Viewed a review request that had its owner changed before this patch

  was applied. The user was determined correctly for the change

  descriptions.