Add OpenSearch support (/opensearch.xml).

Add https://danbooru.donmai.us/opensearch.xml. This file tells browsers
how to perform searches on Danbooru.

In Chrome, this lets you type "danb<tab>" in the address bar to perform
a search on Danbooru.

In Firefox, you have to click the "..." icon next to the address bar,
then choose "Add Search Engine". After that, you can search Danbooru
from the address bar.

Ref:

* http://dev.chromium.org/tab-to-search
* https://developer.mozilla.org/en-US/docs/Web/OpenSearch
* https://github.com/dewitt/opensearch/blob/master/opensearch-1-1-draft-6.md
* https://en.wikipedia.org/wiki/OpenSearch

app/views/layouts/default.html.erb

@@ -6,6 +6,7 @@
   <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon">
   <%= render "meta_links", collection: @current_item %>
   <%= tag.link rel: "canonical", href: canonical_url %>
+  <%= tag.link rel: "search", type: "application/opensearchdescription+xml", href: opensearch_url(format: :xml, version: 1), title: "Search posts" %>
 
   <%= csrf_meta_tag %>
   <% unless CurrentUser.enable_desktop_mode? %>