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.
I think it would be more idiomatic to do

```
self.assertIn("errors", content)
self.assertNotIn("users", content)
```