Limit courses listing to only the instructor's courses

**Is your feature request related to a problem? Please describe.**
Currently instructors can view (but not access) courses they are not instructors for, which the site owner might not want them to see or know about.

**Describe the solution you'd like**
Limit the courses displayed to only the courses the instructor has access to.

**Alternatives considered**
We might want to just hide the Courses listing in the back-end completely, and show on the front-end instead as part of a new dashboard.