Bring back ANNOTATEMORE support

[Samir Aguiar]

Hi all,

We are currently upgrading to 3.0.8, but 3.1.x will probably be stable
by the time we are finished. From what I could see ANNOTATEMORE support
was dropped in 3.1.

Would it possible to bring it back and maybe make it opt-in in
imapd.conf? Our groupware clients depend heavily on it and it would take
a while until we can fully deprecate it.

I ran some tests and apparently reverting #26fbf999312 seems to be
enough, but please correct me if I'm wrong.

Thank you in advance.

Kind regards,
Samir Aguiar