Acknowledge an alert's current firing

Acknowledges the problem behind the alert: its incident when the firing is in one, else the current firing of one group (or every firing group with all_groups). Repeat notifications stop until it clears; a level increase still notifies. Allowed for root, admins and editors (alert edit access with OpenFGA on) and members of a team the problem pages. Every on-call page of the problem stops, unless another group still fires unacknowledged. take_over moves an acknowledgement held by someone else to the caller.