Add missing checks for user existense in GroupsController.accept/rejectRequest, unsubscribeFromGroup.
Add missing checks for user existense in GroupsController.accept/rejectRequest, unsubscribeFromGroup.

FreeFeed/freefeed-server,SiTLar/freefeed-server,FreeFeed/freefeed-server,golozubov/freefeed-server,golozubov/freefeed-server,SiTLar/freefeed-server