Refactors Element#belonging_cellnames

Very complex code just for the draggable trash items...
Use ActiveModel::Model for Message model.
Fixes page etag calculation.

This page etag should always be calculated dependent from current user (if any), not only if the user can visit Alchemy backend. This fixes issues with apps having own user models.
Build against Rails 4.1.0.rc2
Use userstamp 2.1.0 for Rails 4.1.0 compatibility.
rc5