[Dashboard] Change default empty dashboard message to mention All Courses

[jlubkinchavez]

When a student has no active courses, the dashboard has a message that reads: "Welcome to Canvas! You don’t have any courses, so this page won’t be very exciting for now. Once you’ve created or signed up for courses, you’ll start to see conversations from all your classes." They see the same message whether they have never had courses in Canvas or if they have had access prior and retain read-only access to those inactive courses. This idea is to update the default message to remind students that, if applicable, they can access past courses that are in read-only status by going to Courses>All Courses.