Proposed fix & changes to GlobalPagePermissions

This pull request should cover both the fixing & testing of the problem in #1120; it goes slightly further than that and refactors some other usages to be consistent with the logic the admin help_text suggests, but the coverage output seems to indicate that those changes are already caught by other tests. It introduces a new manager, and a new set test case set.

I'm open to ways to improve or refactor it further, should it be deemed required.

Though git thinks there's 4 relevant commits, it looks like two of those will be merged by some strategy or other.