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.
It's not really for indirection, it's to match what the "real" (factory) course looks like.  Course ids are really opaque keys, from now on.