Clear a request's events when deleting the request

[max-moser]

Problem statement

Deleting a request leaves orphan events

Right now, deleting a request does not touch the events (e.g. comments and action-log events) that are associated with the request.
This means that deleting a request will leave a bunch of "orphan" events in the system.
They are still available in the database, and also in the search engine.

Orphans are awkward to handle

Those are especially awkward to look up, since the event service methods usually try to resolve the associated request (which won't work because it's deleted) before trying to look up the events themselves.

Possible solution: Add a service component with a delete hook

Currently, the request service doesn't take events into consideration for delete(), and none of the service components have a hook for delete.
Perhaps we could add a service component that implements such a hook and deletes all associated events.