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.
We shouldn't rely on the user specifically having the Student role, and this can be rolled into a single query:
```
if Role.objects.get(course_id=course_id, users__username=username).exists()
    user_objs.append(...)
```