🐛(frontend) fix enroll action validation

[Tiago-Salles]

Detected the following two bugs on the course detail page:

Enrollment validation

To allows the user to enroll a course, the conditions need to validate if the course is opened, and then show it as available. The Priority enum value ARCHIVED_OPEN does not correspond to this condition.

Enroll action component

The enroll action is being treated as link which does not seem correct in terms of accessibility and from the perspective of call to action concept.