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/controllers/autocomplete_controller.rb

@@ -0,0 +1,16 @@
+class AutocompleteController < ApplicationController
+  respond_to :xml, :json
+
+  def index
+    @tags = Tag.names_matches_with_aliases(params[:query], params.fetch(:limit, 10).to_i)
+
+    if request.variant.opensearch?
+      expires_in 1.hour
+      results = [params[:query], @tags.map(&:pretty_name)]
+      respond_with(results)
+    else
+      # XXX
+      respond_with(@tags.map(&:attributes))
+    end
+  end
+end

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>

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