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/runtests.py reviewboard.datagrids.tests.