[6.x] Update authorization / permissions

[duncanmcclean]

This pull request:

• Only applies our auth logic to Statamic permissions, which prevents us from interfering with custom auth gates.
• Moves our auth logic from Gate::before to after, enabling users to more easily deny access to things.
• Adds super user checks to before in most Statamic policies to continue to avoid overhead when users are super.

Closes #8337.
Closes #10832.

[jasonvarga]

Since we're checking whether an ability is a Statamic one, I had to add a way to get all the resolved permissions. The Permission::all() you had there wouldn't work since it would include placeholders. e.g. edit {collection} entries. The ability passed would be edit blog entries. Permission::flattened() will now resolve all of them.