Add main and utility group menus

[stephen-cox]

Part of localgovdrupal/localgov_microsites#39

[ekes]

I was going to suggest adding the default admin permissions to this PR. But I'm going to spin it out into a new issue #26

At the moment the menu content menu only exposes permissions for the menu itself (confusingly identified as for one menu when it's all the menus) and doesn't expose any permission for adding menu items (and uses the rather far-reaching Administer group permission for this).

So approving as all the rest works, and we need to sort permissions elsewhere.

[ekes]

see #26 for follow-up.