Introduce PositionedFile Decorator

Let `Page#thumbnail_file` and `Entry#thumbnail_file` return
`PositionedFile` objects which provide a `thumbnail_url` and also
background positioning information. The positions are either read from
the sharing image or the corresponding file positions of the page
thumbnail candidate.
Merge branch 'positioned-file' into edge-hosted

Conflicts:
	app/models/pageflow/positioned_file.rb