Fix some bugs
Update Grocery_CRUD to a modified version

The modified version implements callback_can_edit as
steveoc has implemented it before.
Adapt models and mappers to new database structure

Document_models and Document_list_models can now have
several admins. Creator and admins are loaded as User_models.
Add handling of possible concurrent edits, add preview, autogenerate explicitId

- Only one user at a time can edit an entry now
- Value for field explicitId can now be generated
- A (mock) preview field shows, how the bibliographic entry of
a document will look.
Add handling if a list doesn't exist.
Fix bug in check_edit_permisssions_documentlist
Don't write fields created and creator to db.
Merge branch 'crud' of https://github.com/ELK-Phil-UZH/roselit into crud

Merge was necessary because several people have been working in the
branch crud.

Conflicts:
	application/libraries/crud_service.php