"Awaiting" shows cases that are "approved" or "rejected" if submitter was notified

Travel request currently sends an email to the submitter when a case is approved or rejected. As a result, the submitter is "assigned" to the case in the "Approved" or "Rejected" state, so it shows up in the `/awaiting` endpoint. I think the resolution is for that query to exclude cases whose states have no actions.

Start at the implementation of the `/awaiting` endpoint and its case query. Inspect how case states expose available actions, using the approved and rejected notification flow described in the issue. Done means cases with no actions are excluded while actionable assigned cases still appear.

索引モデルが issue の本文から書いたものです。