Handle guest access [1/2?] (#2872)

Needs https://github.com/matrix-org/sytest/pull/1315, as otherwise the
membership events aren't persisted yet when hitting `/state` after
kicking guest users.

Makes the following tests pass:
```
Guest users denied access over federation if guest access prohibited
Guest users are kicked from guest_access rooms on revocation of guest_access
Guest users are kicked from guest_access rooms on revocation of guest_access over federation
```

Todo (in a follow up PR):
- Restrict access to CS API Endpoints as per
https://spec.matrix.org/v1.4/client-server-api/#client-behaviour-14

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

clientapi/routing/joinroom.go

@@ -37,6 +37,7 @@ func JoinRoomByIDOrAlias(
 	joinReq := roomserverAPI.PerformJoinRequest{
 		RoomIDOrAlias: roomIDOrAlias,
 		UserID:        device.UserID,
+		IsGuest:       device.AccountType == api.AccountTypeGuest,
 		Content:       map[string]interface{}{},
 	}
 	joinRes := roomserverAPI.PerformJoinResponse{}
@@ -84,7 +85,14 @@ func JoinRoomByIDOrAlias(
 		if err := rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes); err != nil {
 			done <- jsonerror.InternalAPIError(req.Context(), err)
 		} else if joinRes.Error != nil {
-			done <- joinRes.Error.JSONResponse()
+			if joinRes.Error.Code == roomserverAPI.PerformErrorNotAllowed && device.AccountType == api.AccountTypeGuest {
+				done <- util.JSONResponse{
+					Code: http.StatusForbidden,
+					JSON: jsonerror.GuestAccessForbidden(joinRes.Error.Msg),
+				}
+			} else {
+				done <- joinRes.Error.JSONResponse()
+			}
 		} else {
 			done <- util.JSONResponse{
 				Code: http.StatusOK,