Adds specs for Admin::EssencesHelper#essence_picture_thumbnail
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.
Merge pull request #421 from robinboening/admin_essences_helper_specs

Adds specs for Admin::EssencesHelper#essence_picture_thumbnail
Removes unused method from admin base_helper and adds spec for #translations_for_select method.
Merge pull request #422 from robinboening/removing_not_used_helpers

Removes not used method from admin base_helper.