Add ability to use nested only parameter

- The only string works much the same as before with its comma separation
-- Nested includes are indicated with square brackets "[ ]"
-- The nested include is the value immediately preceding the square brackets
-- The only string is the comma separated string inside those brackets
- Default includes are split between format types when necessary
-- This prevents unnecessary includes from being added on page load
- Available includes are those items which are allowed to be accessible to the user
-- Some aren't because they are sensitive, such as the creator of a flag
-- Some aren't because the number of associated items is too large
- The amount of times the same model can be included to prevent recursions
-- One exception is the root model may include the same model once
--- e.g. the user model can include the inviter which is also the user model
-- Another exception is if the include is a has_many association
--- e.g. artist urls can include the artist, and then artist urls again

app/models/post_archive.rb

@@ -274,5 +274,9 @@ class PostArchive < ApplicationRecord
     super + [:obsolete_added_tags, :obsolete_removed_tags, :unchanged_tags, :updater_name]
   end
 
+  def self.available_includes
+    [:updater, :post]
+  end
+
   memoize :previous, :tag_array, :changes, :added_tags_with_fields, :removed_tags_with_fields, :obsolete_removed_tags, :obsolete_added_tags, :unchanged_tags
 end