Make name, capped_size readonly in ORM and admin

Also prevent deletion of objects referenced by datasets

Because it would be a lot of work to be able to change these in Backdrop
right now - we're going "create only"

It should not be possible to delete data-groups and data-types used by data-sets.

We do validation in clean() to check that "readonly" fields haven't
changed.
Remove delete admin permission and delete action

This removes the ability to delete data-sets from the admin GUI.
Merge pull request #39 from alphagov/restrict-data-set-updating-squashed

Make name, capped_size readonly in ORM and admin