Make text on register button on About page conditional.

 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.
Merge pull request #4418 from Stanford-Online/dcadams/make_registration_button_conditional

Make text on register button on About page conditional.