Add a per-user setting to hide followers count

This is only available if the instance-wide setting isn't set and allows
people to hide their own followers count. This does not hide others' to
them.

app/serializers/rest/account_serializer.rb

@@ -53,6 +53,6 @@ class REST::AccountSerializer < ActiveModel::Serializer
   end
 
   def followers_count
-    Setting.hide_followers_count ? -1 : object.followers_count
+    (Setting.hide_followers_count || object.user&.setting_hide_followers_count) ? -1 : object.followers_count
   end
 end