R1:174dd220dfe6

Disable mentions in nonpublishing repositories

Summary:
Ref T6516. Although this behavior is somewhat-arguable as desirable, I think it's less surprising and more consistent to disable mentions when a repository is publishing.

In particular, if you import a repository developed on another Phabricator install, this stops all the `T123` in commit messages from creating mentions on your unrelated `T123` tasks.

We already disable autoclose, so `Closes T123` and `Ref T123` already have no effect, but a bare…