Allow Activity registers actions of share and unshare for circles

[fgsl]

Circles does not emit signals to actions of share and unshare, so Activity app does not register these actions.

First I tried to follow same implementation of groups, and submit changes for server and activity projects, but as recommended by @nickvergessen [1] and @rullzer in [2], I will change only Circles app, using part of signals implemented in [3], that is not integrated yet.

[1] nextcloud/activity#269 (comment)
[2] nextcloud/server#9463 (review)
[3] #163