Add OpenSearch suggestion support.

Add autocomplete support when searching Danbooru from the Chrome address
bar. If you type "danb<tab>" in the address bar then search for a tag,
then autocomplete results from Danbooru will appear as search
suggestions in Chrome.

Note that the "Autocomplete searches and URLs" Chrome setting must be
enabled for this to work.

Ref:

* http://dev.chromium.org/tab-to-search
* https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Supporting_search_suggestions_in_search_plugins
* https://github.com/dewitt/opensearch/blob/master/mediawiki/Specifications/OpenSearch/Extensions/Suggestions/1.1/Draft%201.wiki

app/views/static/opensearch.xml.erb

@@ -4,4 +4,5 @@
   <Description><%= Danbooru.config.app_name %> search</Description>
   <Image height="16" width="16" type="image/x-icon"><%= root_url %>favicon.ico</Image>
   <Url type="text/html" template="<%= posts_url %>?tags={searchTerms}&amp;utm_source=opensearch"/>
+  <Url type="application/x-suggestions+json" template="<%= autocomplete_index_url(format: :json) %>?query={searchTerms}&amp;type=tags&amp;variant=opensearch"/>
 </OpenSearchDescription>