Change "Home" link to "Courses" when user has no active enrollments & groups

When the current user has no active enrollments or groups the Courses dropdown changes to a Home link. In doing so there is no way to get to the Courses page to see prior enrollments without knowing the direct URL. This mod changes the Home link to a link to /courses.

Test plan:
1. Log in as a user with no active enrollments (see me for a test user ID)
2. Verify that Courses is in the top nav bar and is a link to /courses
Add text to courses index page when no current enrollments

When a user has no current or past enrollments, or current or past groups, their /courses page is completely blank. To improve the user experience, there should be some explanatory text on the page.

Test plan:
1. Log in as a user with no current or past enrollments or groups (see me for a test user ID)
2. Verify that the "You aren't enrolled in any current courses" text is displayed.
Add courses.index.no_courses_message to en.yml