Add has_file flag to Ontology.
Define own walk order.
Mark ontology as having no file after deletion.
Only read cached `has_file` when on HEAD.
Add spec on file deletion.
Add controller action and route to delete a file.
Add views for file deletion.
Obey hound.
Add deleted label, link to last file and helpers.
Obey hound.
Merge branch '617-handle_file_deletion' into develop

Conflicts:
	app/helpers/files_helper.rb
	app/views/files/show.html.haml