Change the button "enroll" to "unenroll" when user is enrolled in course

[YaroslavKSE]

Acceptance criteria:

1. The button should change from "enroll" to "unenroll" when user is enrolled in course while looking at course details
2. The message should display "Are you sure that you want to unrell from course". If the user clicks "yes" then the message shows "You were successfully unenrolled from course". If the user clicks "no" then the message window closes.