Rename "locked" to "manuallyApprovesFollowers" in ActivityPub (#4779)

See: <https://www.w3.org/wiki/Activity_Streams_extensions#as:manuallyApprovesFollowers>

app/serializers/activitypub/actor_serializer.rb

@@ -6,7 +6,7 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   attributes :id, :type, :following, :followers,
              :inbox, :outbox, :shared_inbox,
              :preferred_username, :name, :summary,
-             :url, :locked
+             :url, :manually_approves_followers
 
   has_one :public_key, serializer: ActivityPub::PublicKeySerializer
 
@@ -90,4 +90,8 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   def header_exists?
     object.header.exists?
   end
+
+  def manually_approves_followers
+    object.locked
+  end
 end