Improve api oembed controller (#3450)

* Add StreamEntryFinder class to parse URLs

* Use StreamEntryFinder and clean up api/oembed controller

spec/controllers/api/oembed_controller_spec.rb

@@ -1,6 +1,8 @@
 require 'rails_helper'
 
 RSpec.describe Api::OEmbedController, type: :controller do
+  render_views
+
   let(:alice)  { Fabricate(:account, username: 'alice') }
   let(:status) { Fabricate(:status, text: 'Hello world', account: alice) }