Render "Fetch source data" box html server-side.

Instead of calling /sources.json and rendering the "Fetch source data"
box client-side in Javascript, call /sources.js so we can render the
html server-side.

app/controllers/sources_controller.rb

@@ -1,5 +1,5 @@
 class SourcesController < ApplicationController
-  respond_to :json, :xml
+  respond_to :js, :json, :xml
 
   def show
     @source = Sources::Strategies.find(params[:url], params[:ref])