Cannot change permissions on a finalised model

[jamesrwelch]

Describe the bug
The 'Add Group' button does not appear on the permissions modal window when viewing a finalised model.
I believe any changes to the 'publicly readable' and 'readable by authenticated users' are also not persisted.

Expected Behaviour
You should be able to change the permissions of a finalised model.

[pjmonks]

For comparison, here is what the group permissions looks like for a draft data model:

This is what it looks like for a finalised one:

So the "Add Group" button is definitely missing and should be put back somehow.