Fix favicon on HiDPI displays

assets/javascripts/app/config.coffee.erb

@@ -13,5 +13,6 @@ app.config =
   version: <%= Time.now.to_i %>
   release: <%= Time.now.utc.httpdate.to_json %>
   mathml_stylesheet: '<%= App.cdn_origin %>/mathml.css'
+  favicon_spritesheet: '<%= image_path('sprites/docs.png') %>'
   service_worker_path: '/service-worker.js'
   service_worker_enabled: <%= App.environment == :production || ENV['ENABLE_SERVICE_WORKER'] == 'true' %>

assets/javascripts/lib/favicon.coffee

@@ -30,8 +30,8 @@ withImage = (url, action) ->
 
   styles = window.getComputedStyle($("._icon-#{doc.slug.split('~')[0]}"), ':before')
 
-  bgUrl = styles['background-image'].slice(5, -2)
+  bgUrl = app.config.favicon_spritesheet
-  sourceSize = if bgUrl.includes('@2x') then 32 else 16
+  sourceSize = 16
   sourceX = Math.abs(parseInt(styles['background-position-x'].slice(0, -2)))
   sourceY = Math.abs(parseInt(styles['background-position-y'].slice(0, -2)))