CoursePress should allow instructors to see course content in the courses where enrolled

I submitted a separate ticket about no longer being able to see Courses You Manage, but I think there is a more general issue in CoursePress where instructors should be able to see the course content and access anything that the students can access...and more.

To that end, I have two suggestions on how the Instructor role should work:

1. By default, the instructor should be able to access the course on the front end of the site. Currently, unless they are also enrolled as a student, they can't see the course. This limits their ability both to respond to student questions about the course as well as their ability to interact with the student in the discussion board.

2. In the instructor capabilities settings, there is a limit on the instructor's ability to view the units. As you can probably tell from the above, I don't think there should be a limit to view the units in the courses where the instructor is enrolled, but if there is a valid use case for this, there should at least be a setting that allows the admin to give the instructor the privilege to view their own courses, without also giving them the privilege to view everyone else's courses. Currently, if I check "View units in every course", the instructor sees all of the units in all courses and there is no ability to limit their ability to just view the units in the courses where they are enrolled.