Make text on register button on About page conditional.

This PR replaces PR(#3654) which I'm closing. 

Added a new advanced setting; invitation_only

 If invitation_only display "Enrollment in this course is by invitation only"
 Otherwise, if outside of enrollment period display "Enrollment is Closed"

 This issue is logged at LMS-2670

 Also fixed in this PR is issue LMS-2717; even though the Register button was greyed out,
 clicking it sent one to the registration page.
This is the same pattern followed elsewhere in access.py. The debug function only logs if a particular configuration flag is turned on.