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

test/functional/static_controller_test.rb

@@ -63,4 +63,11 @@ class StaticControllerTest < ActionDispatch::IntegrationTest
       assert_response :success
     end
   end
+
+  context "opensearch action" do
+    should "work" do
+      get opensearch_path, as: :xml
+      assert_response :success
+    end
+  end
 end