Use `non-stupid-digest-assets` gem to have non digest versions of compiled assets. Closes #419

As sprocket-rails does not compile the non digests versions of the assets anymore, we have to use the `non-stupid-digest-assets` gem to get them back and to have Tinymce working again.

NOTE: Unless a new version of the gem was released we have to use my fork, to also have the gzip'ed versions of non digest assets.
Move the Tinymce content.css file into proper location.