Implement server notices (#2180)

* Add server_notices config

* Disallow rejecting "server notice" invites

* Update config

* Slightly refactor sendEvent and CreateRoom so it can be reused

* Implement unspecced server notices

* Validate the request

* Set the user api when starting

* Rename function/variables

* Update comments

* Update config

* Set the avatar on account creation

* Update test

* Only create the account when starting
Only add routes if sever notices are enabled

* Use reserver username
Check that we actually got roomData

* Add check for admin account
Enable server notices for CI
Return same values as Synapse

* Add custom error for rejecting server notice invite

* Move building an invite to it's own function, for reusability

* Don't create new rooms, use the existing one (follow Synapse behavior)

Co-authored-by: kegsay <kegan@matrix.org>

clientapi/routing/leaveroom.go

@@ -38,6 +38,12 @@ func LeaveRoomByID(
 
 	// Ask the roomserver to perform the leave.
 	if err := rsAPI.PerformLeave(req.Context(), &leaveReq, &leaveRes); err != nil {
+		if leaveRes.Code != 0 {
+			return util.JSONResponse{
+				Code: leaveRes.Code,
+				JSON: jsonerror.LeaveServerNoticeError(),
+			}
+		}
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
 			JSON: jsonerror.Unknown(err.Error()),