Fix MRF policies to also work with Update

Fix MRF policies to also work with Update

Objects who got updated would just pass through several of the MRF policies, undoing moderation in some situations.
In the relevant cases we now check not only for Create activities, but also Update activities.

I checked which ones checked explicitly on type Create using `grep '"type" => "Create"' lib/pleroma/web/activity_pub/mrf/*`.

The following from that list have not been changed:
* lib/pleroma/web/activity_pub/mrf/follow_bot_policy.ex
    * Not relevant for moderation
* lib/pleroma/web/activity_pub/mrf/keyword_policy.ex
    * Already had a test for Update
* lib/pleroma/web/activity_pub/mrf/object_age_policy.ex
    * In practice only relevant when fetching old objects (e.g. through Like or Announce). These are always wrapped in a Create.
* lib/pleroma/web/activity_pub/mrf/reject_non_public.ex
    * We don't allow changing scope with Update, so not relevant here