wiki pages: drop creator and updater fields.

Drop the creator_id and updater_id fields from wiki pages. These fields
had several issues:

* The creator_id field was inconsistent with the wiki_page_versions
  table. Apparently during the migration to Danbooru 2 in 2012-2013 the
  creator_id field got reset to whoever last updated the wiki at that
  point in time.

* Saving a wiki would set the updater_id even when nothing actually
  changed. This also caused the updated_at timestamp to get bumped.
  Because of this, anything that saved a wiki, including things like
  creating aliases or implications, would bump the updater_id and
  updated_at even though the wiki didn't actually change. This meant
  these fields weren't consistent with the wiki_page_versions history.

Changes:

* Remove `creator_name` field from the /wiki_pages.json API.
* Remove creator name search option from /wiki_pages/search.

db/migrate/20191024194544_drop_creator_and_updater_from_wiki_pages.rb

@@ -0,0 +1,6 @@
+class DropCreatorAndUpdaterFromWikiPages < ActiveRecord::Migration[6.0]
+  def change
+    remove_column :wiki_pages, :creator_id, :integer
+    remove_column :wiki_pages, :updater_id, :integer
+  end
+end