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/api/perform.go

@@ -159,3 +159,11 @@ type PerformPublishResponse struct {
 	// If non-nil, the publish request failed. Contains more information why it failed.
 	Error *PerformError
 }
+
+// PerformForgetRequest is a request to PerformForget
+type PerformForgetRequest struct {
+	RoomID string `json:"room_id"`
+	UserID string `json:"user_id"`
+}
+
+type PerformForgetResponse struct{}