Prevent moderators from editing group title, description and type

[seanh]

We want a group's moderators to be able to access the group's edit-members page (/groups/{pubid}/edit/members) because (once we've implemented #9136) moderators should be able to use this page to downgrade their own role to member. (Moderators can't change anyone else's role other than their own, but IIRC they can remove plain members from the group.)

But we don't want moderators to be able to edit the group's other settings (name, description, type, etc) on the /groups/{pubid}/edit page. Currently moderators can access this page and edit these settings.

Design and implement a way for moderators to have access to the /groups/{pubid}/edit/members page but not to the /groups/{pubid}/edit (or to have read-only access to the /groups/{pubid}/edit page).