align to/cc addressing pattern with friendica, hubzilla instead of mastodon

This MR aligns our addressing pattern for mentions with friendica and hubzilla instead of Mastodon.

There is no effect with Mastodon-like platforms with this change but it solves an important problem concerning Friendica and Hubzilla compatibility.

Now the mentions are calculated based on to field and any tagged Mention items. Accordingly, we also re-unify handling of Announce activities which was separate before.

Closes: #341 (closed).