Add a new permissions name 'visible_in_catalog' to the access.py file to...

... allow for an explicit 'show in catalog' setting so that White Label sites can show/hide courses outside of Enrollment/Start time windows
Note to reviewers. This change to fix the course about page for paid course registrations. The previous implementation had the if-conditionals in an order such that the purchase button was always visible even if the course had a closed enrollment or other enrollment gating function.