Make fullname column sortable on /users/ page

Review Request #10277 — Created Oct. 25, 2018 and updated — Latest diff uploaded

Information

Review Board
release-4.0.x
2555e5a...

Reviewers

This fix makes the FullNameColumn column of the /users/ page
sortable. Previously, only UsernameColumn was sortable.

In order to implement this, an override method for get_sort_field()
was added to FullNameColumn which returns the first_name and
last_name as a tuple. A related change in the Djblets side enables
handling of tuples as returns from get_sort_field().

Two unit tests were added to test sorting of the test users in both
ascending and descending order using custom first and last names.

All tests pass for Review Board, except "Testing PerforceTool.connect
sets required client args", which can be disregarded for now.