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

test/functional/autocomplete_controller_test.rb

@@ -0,0 +1,17 @@
+require "test_helper"
+
+class AutocompleteControllerTest < ActionDispatch::IntegrationTest
+  context "Autocomplete controller" do
+    context "index action" do
+      setup do
+        create(:tag, name: "azur_lane")
+      end
+
+      should "work for opensearch queries" do
+        get autocomplete_index_path(query: "azur", variant: "opensearch"), as: :json
+        assert_response :success
+        assert_equal(["azur", ["azur lane"]], response.parsed_body)
+      end
+    end
+  end
+end