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.

spec/controllers/metadata_controller_spec.rb

@@ -0,0 +1,28 @@
+require 'rails_helper'
+
+RSpec.describe MetadataController do
+  describe 'manifest.json' do
+    it 'returns the right output' do
+      title = 'MyApp'
+      SiteSetting.title = title
+      get :manifest
+      expect(response.body).to include(title)
+      expect(response.content_type).to eq('application/json')
+    end
+  end
+
+  describe 'opensearch.xml' do
+    it 'returns the right output' do
+      title = 'MyApp'
+      favicon_path = '/uploads/something/23432.png'
+      SiteSetting.title = title
+      SiteSetting.favicon_url = favicon_path
+      get :opensearch, format: :xml
+      expect(response.body).to include(title)
+      expect(response.body).to include("/search?q={searchTerms}")
+      expect(response.body).to include('image/png')
+      expect(response.body).to include(favicon_path)
+      expect(response.content_type).to eq('application/xml')
+    end
+  end
+end