Add a key for document.type.document_collection.*
Make it clearer that we use DocColGroupsController#delete

Avoids confusion about when before_filters are and
are not needed.
Rename title in doc collection groups index
[DC] Rename session key read as well as write
[DC] Remove unused HTML ID.
[DC] Copy change in DC group delete page.
[DC] Remove redundant check on #published_document_collections

This is always an array from the #map#compact.