Add ReviewRequest.get_last_activity.

This change adds a new get_last_activity() function to ReviewRequest.
Previously we've had different bits of code that attempted to compute the
last activity time, and they did it differently. This caused some bugs,
such as caching issues and excess update notification bubbles.

Now we have one single function that returns the last update timestamp and
the object that was updated. The review request page now uses just the
timestamp for its ETag and last_activity_time variable, and the last-update
API call makes use of the object to provide more detailed information on
the update.

Made some updates to a review request I was on. Saw that the update bubbles appeared as normal.

Updated a review and clicked the Update link to reload the page on the notification bubble. Didn't see a notification after that.