Updating the paginator in datagrid to allow alphabetic querying to be turned on by providing the DataGrid object with the current_letter parameter (default none).
If the current_letter is provided to the datagrid (usually by some child grid calling the super initializer) the datagrid will simply load a second template case added paginator.html. 
It is the job of the child class (UserGrid in reviewboard) to process the URI, perform filtering on the queryset, and handle errors. 

Unit tests all pass

All button cases manually tested. (Forward, back, first and last arrows, each letter). 

Verified that  changes to not affect any other module that inherits from datagrid (such as the groups list, or the all review requests list, the current 'master' of reviewboard with no changes to the userGrid).