FEATURE: Add /search discovery

The opensearch.xml results in a "site search engine" being added to
Chrome, while the sitelinks search tag results in "Search this website"
being added to Google Search.

app/controllers/metadata_controller.rb

@@ -0,0 +1,21 @@
+class MetadataController < ApplicationController
+  layout false
+  skip_before_filter :preload_json, :check_xhr, :redirect_to_login_if_required
+
+  def manifest
+    manifest = {
+      short_name: SiteSetting.title,
+      display: 'standalone',
+      orientation: 'portrait',
+      start_url: "#{Discourse.base_uri}/",
+      background_color: "##{ColorScheme.hex_for_name('secondary')}",
+      theme_color: "##{ColorScheme.hex_for_name('header_background')}"
+    }
+
+    render json: manifest.to_json
+  end
+
+  def opensearch
+    render file: "#{Rails.root}/app/views/metadata/opensearch.xml"
+  end
+end