"r?" silently does nothing when the user is not a reviewer

[RalfJung]

In Miri, this is our configuration. If we now do r? @user, then nothing happens -- nobody gets assigned, but also no error is shown.

[apiraino]

Tried to look at the triagebot logs but don't have enough visibility. I suspect something fails when the triagebot tries to pick someone from this this file but I don't see errors, this could be why the silent failure.

[ehuss]

r? requires an assign.owners table to work. This was required to support the transition from highfive (so two bots didn't respond to every PR). Otherwise it is ignored.

It may be possible to do something like have an empty owners table to enable it and support something like #1806, but currently that is not supported. In the meantime, you can do the more explicit syntax of @rustbot assign @user

[Kobzol]

This should now work (#1937).