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/views/wiki_pages/index.html.erb

@@ -14,7 +14,7 @@
       <% @wiki_pages.each do |wiki_page| %>
         <tr>
           <td class="category-<%= wiki_page.category_name %>"><%= link_to wiki_page.pretty_title, wiki_page_path(wiki_page, :noredirect => 1) %></td>
-          <td><%= wiki_page.updated_at.strftime("%Y-%m-%d %H:%M") %> by <%= h link_to_user wiki_page.updater %></td>
+          <td><%= time_ago_in_words_tagged(wiki_page.updated_at) %></td>
         </tr>
       <% end %>
     </tbody>

app/views/wiki_pages/search.html.erb

@@ -3,7 +3,6 @@
     <%= search_form_for(wiki_pages_path) do |f| %>
       <%= f.input :title, hint: "Use * for wildcard searches", input_html: { "data-autocomplete": "wiki-page" } %>
       <%= f.input :other_names_match, label: "Other names", hint: "Use * for wildcard searches" %>
-      <%= f.input :creator_name, input_html: { "data-autocomplete": "user" } %>
       <%= f.input :body_matches, label: "Body" %>
       <%= f.input :other_names_present, as: :select %>
       <%= f.input :hide_deleted, as: :select, include_blank: false %>