Implement forgetting about rooms (#1572)

* Add basic storage methods

* Add internal api handler

* Add check for forgotten room

* Add /rooms/{roomID}/forget endpoint

* Add missing rsAPI method

* Remove unused parameters

* Add passing tests

Signed-off-by: Till Faelligen <tfaelligen@gmail.com>

* Add missing file

* Add postgres migration

* Add sqlite migration

* Use Forgetter to forget room

* Remove empty line

* Update HTTP status codes

It looks like the spec calls for these to be 400, rather than 403: https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-rooms-roomid-forget

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

roomserver/internal/perform/perform_invite.go

@@ -86,7 +86,7 @@ func (r *Inviter) PerformInvite(
 
 	var isAlreadyJoined bool
 	if info != nil {
-		_, isAlreadyJoined, err = r.DB.GetMembership(ctx, info.RoomNID, *event.StateKey())
+		_, isAlreadyJoined, _, err = r.DB.GetMembership(ctx, info.RoomNID, *event.StateKey())
 		if err != nil {
 			return nil, fmt.Errorf("r.DB.GetMembership: %w", err)
 		}