Add a "Last Updated By" column to ReviewBoard dashboard

Review Request #11524 — Created March 14, 2021 and updated — Latest diff uploaded

Review Board
reviewboard, students

Currently, the last updated column only shows the timestamp
of last activity that has occurred. It is desireable to show
the user who is responsible for the last activity as well.
Therefore, we need to add another column "Last Updated By" to
the dashboard.

Manual testing:
1. On the UI, activate the new Last Updated By column by selecting it.
2. It should show the user (linkable to the user's profile) that is
responsible for the last update.
Tested on Safari, Chrome, and Firefox.

Automated unit testing:
1. Run ./tests/ reviewboard.datagrids.tests.