Add an asset store to put assets under Case
Allow augmentation with initial undertakings

* Also change Case/CaseStore to use ActiveModel serializers
  (rather than roll-your-own)
Let asset cope with an original_url as a URI::HTTP
Don't kill the crawl, by, I dunno, removing all the links or anything.
Move the crawler into its own class

* Makes looking up the referer tree slightly cleaner