This makes us the original sender's intent to redact messages,
so that we do not require an elevated PL to handle message deletion.

…rmine a more fitting user

We may produce multiple Matrix events per Slack events, if the Slack events contains text with attachments.
This should have been forbidden with a DB constraint, but for some reason it happened regardless.

This updates the constraint to match reality, differentiating Matrix events by the type of incoming Slack event.
For backwards compatibility reasons getEventBySlackId() returns the "main" Matrix event.

…zik/one-to-many-events

Messages with a message_changed will have the same `ts` as the original message, but a different `event_ts`.
We now check both when considering whether or not to drop a message, and log our reasons regardless.

We now track which attachments was linked to which Matrix message,
and support both removing them individually and dropping the whole event "tree".