Ziafazal/add filters to user api 

Created a user list api to return paginated list of users with ability to filter by email or username. It supports MCKIN-1262 and MCKIN-1263.
@chrisndodge @mattdrayer Please review.
Dumb Django question -- when is this queryset evaluated?  Is it safe to locate the line here or should it be moved into the get() method after the filter check?