Implement a working method for sharing a link for the contents of share

rooms.  It's different than the link-determining technique for the contents
of storage.  The technique for the latter will fail characteristically when
tried for the former, so we can fallback to the technique for the former in
that case.

(One nuance we do not provide is indicating in the link sharing message that
a password will be required, for the contents of password-protected share
rooms.  Providing for that would require more code, including another ajax request.)