Improve the ACL UI

[TimWolla]

Often the board permissions are configured unnecessarily complex, not making use of inheritance and uselessly specifying permissions when the group permission is already appropriate.

The ACL UI likely is at fault and should be improved:

• [x] Remove the “Full Access” checkbox: It's there and it's attractive to deny access to “Everyone” or “Guests”. However it's almost never the right choice, because it requires configuring all permissions for the groups that should have access instead of inheriting the group permissions.
• [ ] Change the UI from two checkboxes (that cannot actually be checked both) to a real radio button with an explicit “Inherit” option that works as if no checkbox would be checked. This should make it clearer that permissions are inherited from somewhere else.