implement forums endpoint for searching users.

Very limited functionality at this stage - only finds exact matches (so there will only be 0 or 1 result).

@gwprice 

@aboudreault @antoviaque FYI - new endpoint.  frontend will be added in a subsequent PR.
cool, doing exists() now.  i'm about to add the check on non-anonymous posts by the user, which may end up making this part unnecessary.