Improve support for aspects/circles (#8950)

* Add silent column to mentions

* Save silent mentions in ActivityPub Create handler and optimize it

Move networking calls out of the database transaction

* Add "limited" visibility level masked as "private" in the API

Unlike DMs, limited statuses are pushed into home feeds. The access
control rules between direct and limited statuses is almost the same,
except for counter and conversation logic

* Ensure silent column is non-null, add spec

* Ensure filters don't check silent mentions for blocks/mutes

As those are "this person is also allowed to see" rather than "this
person is involved", therefore does not warrant filtering

* Clean up code

* Use Status#active_mentions to limit returned mentions

* Fix code style issues

* Use Status#active_mentions in Notification

And remove stream_entry eager-loading from Notification

app/lib/activitypub/activity/create.rb

@@ -28,6 +28,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
     process_status_params
     process_tags
+    process_audience
 
     ApplicationRecord.transaction do
       @status = Status.create!(@params)
@@ -66,6 +67,27 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     end
   end
 
+  def process_audience
+    (as_array(@object['to']) + as_array(@object['cc'])).uniq.each do |audience|
+      next if audience == ActivityPub::TagManager::COLLECTIONS[:public]
+
+      # Unlike with tags, there is no point in resolving accounts we don't already
+      # know here, because silent mentions would only be used for local access
+      # control anyway
+      account = account_from_uri(audience)
+
+      next if account.nil? || @mentions.any? { |mention| mention.account_id == account.id }
+
+      @mentions << Mention.new(account: account, silent: true)
+
+      # If there is at least one silent mention, then the status can be considered
+      # as a limited-audience status, and not strictly a direct message
+      next unless @params[:visibility] == :direct
+
+      @params[:visibility] = :limited
+    end
+  end
+
   def attach_tags(status)
     @tags.each do |tag|
       status.tags << tag
@@ -113,7 +135,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
     return if account.nil?
 
-    @mentions << Mention.new(account: account)
+    @mentions << Mention.new(account: account, silent: false)
   end
 
   def process_emoji(tag)