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).