Cache result of isSuperUser check in permission handling

If you run Confluence without user property caching, it does around twenty database hits per request to check whether the current user is disabled or not. This is the deactivated check in UserAccessor.isSuperUser() method, which gets called for most permission checks in Confluence.

We should cache the result of this check for each request so we only need to check whether the user is disabled once.

This fix will involve moving the isSuperUser check into the PermissionManager so that we don't need to add caching in all the calling code or in the UserAccessor which currently has none.