These changes add a new column to the review request dashboard called "Last Updated By" which displays the user that made the last update to each review request. Similar to the existing "Last Updated" column, which shows the time that the last update was made, this column is updated whenever a review request is created, edited, receives a review, is closed, or reopened.
Changes
"Last Updated By" column

Added a "Last Updated By" column
Added information about the new column to manual
Added tests for the new column

ReviewRequest last activity info functions

ReviewRequest.get_last_activity_info() also returns the user that last updated the review request
Review requests cache their last activity info (last updated object, changedesc, and user) in ReviewRequest.extra_data
Added ReviewRequest.set_last_activity_info(timestamp, updated_object, changedesc, user)
Modified existing uses of ReviewRequest.get_last_activity_info() to take advantage of the user field
Added and modified existing tests of ReviewRequest.get_last_activity_info()

Other

Made the WebAPI ReviewRequestLastUpdateResource return the correct user now that ReviewRequest.get_last_activity_info() includes it

Added tests for the column itself as well as the new last activity-related functions.
Ran all backend tests and observed that all passed.
Performed manual testing of the functionality of the column by creating, editing, reviewing, closing, and reopening review requests and observing how the column changes.