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.

app/controllers/wiki_pages_controller.rb

@@ -16,7 +16,7 @@ class WikiPagesController < ApplicationController
   end
 
   def index
-    @wiki_pages = WikiPage.includes(:creator).paginated_search(params)
+    @wiki_pages = WikiPage.paginated_search(params)
     respond_with(@wiki_pages) do |format|
       format.html do
         if params[:page].nil? || params[:page].to_i == 1