[webui][api] Refactor attributes

This refactors attributes to use use "standard" rails/activerecord features as much as possible. It also:

* introduces 2 new gems: acts_as_list for for the list features of AttribValues and cocoon for handling nested forms
* introduces a basic working interface for attributes in the webui
* fixes some of the features (allowed values, default values)
* moves redirect_back_or_to to the webui_controller (helpers are for views)
* changes the message of WrongRouteForAttribute
* rewords most attrib_type descriptions to be more concise