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.