Suggestion: Disable CMS-based creation of menusets by default

[sminnee]

I expect that the typical use-case of this module is:

• Reference a MenuSet in the template
• Add it to the list of default menusets

In such cases there would be no reason for a user to create or delete menusets - just to add items to the ones that are predefined.

As such, would it make sense to disable create/delete in the CMS by default? Perhaps you could leave those features in dev environments to help people creating templates.

If that's too bold a change, perhaps a config option that lets people easily disable the create/delete features would be better.