GitLab

Anything the app UI can't display is considered a "custom" rule,
and editing is prohibited.  The check was not taking into
consideration monthly events for which the day was specified with
a negative value (e.g. "the last day of every month"), which we
don't handle.

Bug 4522027

Change-Id: Id10e9f6d744cc8da7ac115ee93c2b81bd826cfe2