Prevent moderators from editing group title, description and type

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 https://github.com/hypothesis/h/issues/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).