Resolve #9436 cache favicons

  * Add favicon initializer
  * Add Favicon::Cacher module
  * Add Favicon::Retriever module
  * Remove Page#icon method
  * Remove PagesController#cached_icon
  * Fix Page view test
  * Add FaviconCacherWorker
  * Update seed_oni Rake task to cache favicons when creating new Pages
  * Reduce Sidekiq concurrency
  * Add images/favicons assets directory to repo
  * Add netclerk:favicons:cache Rake task