Make FileReferenceInputView Set Attribute to null

Used to unset which caused the entry sharing image not be resetable.
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 pull request #230 from tf/file-input-reset

Make FileReferenceInputView Set Attribute to null
Merge pull request #231 from tf/positioned-file

Introduce PositionedFile Decorator