User notification tab: 'edit subscriptions' list not populated correctly

Problem/Motivation

Items under edit subscriptions may not display, despite a user having the 'administer users' permission due to an error in the is_visible method on the Notifications_Custom_Subscriptions class.

To reproduce:

1. Add a custom subscription that does not have the show on user registration option set.
2. Try and access the page as a user with 'administer users'

Proposed resolution

The method needs brackets around the ternary operation in the second half of the 'or' statement.