Filter Announce activities with blocked users in cc

Pleroma attempts to filter boosts from blocked users from timelines. However, it does so solely based on the to field of the Announce activities, which works for Pleroma-issued announces, but not Mastodon-issued announces (which ccs the announced content's author instead).

This PR applies the same logic to the cc field. Of course, nothing mandates implementations to actually have the announced content's author anywhere in the audience, so this is not an actual solution. An actual solution would be to check the author of the announced content, but I do not know Pleroma's codebase enough to do it efficiently.

I also do not know Pleroma's codebase enough to write a test for this case (the existing testcase creates the announce object through the CommonAPI code).

Also note that I haven't actually tested this, as I have no Pleroma instance set up.