Reduce usage of LD signatures (#9659)

* Do not LDS-sign Follow, Accept, Reject, Undo, Block

* Do not use LDS for Create activities of private toots

* Minor cleanup

* Ignore unsigned activities instead of misattributing them

* Use status.distributable? instead of querying visibility directly

app/services/activitypub/process_collection_service.rb

@@ -27,7 +27,7 @@ class ActivityPub::ProcessCollectionService < BaseService
   private
 
   def different_actor?
-    @json['actor'].present? && value_or_id(@json['actor']) != @account.uri && @json['signature'].present?
+    @json['actor'].present? && value_or_id(@json['actor']) != @account.uri
   end
 
   def process_items(items)