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/structure.sql

@@ -3206,14 +3206,12 @@ ALTER SEQUENCE public.wiki_page_versions_id_seq OWNED BY public.wiki_page_versio
 
 CREATE TABLE public.wiki_pages (
     id integer NOT NULL,
-    creator_id integer NOT NULL,
     title character varying NOT NULL,
     body text NOT NULL,
     body_index tsvector NOT NULL,
     is_locked boolean DEFAULT false NOT NULL,
     created_at timestamp without time zone,
     updated_at timestamp without time zone,
-    updater_id integer,
     other_names text[] DEFAULT '{}'::text[] NOT NULL,
     is_deleted boolean DEFAULT false NOT NULL
 );
@@ -7408,6 +7406,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20190919175836'),
 ('20190923071044'),
 ('20190926000912'),
-('20191023191749');
+('20191023191749'),
+('20191024194544');