Replace usages of Sensei_Utils::user_started_course in course methods

[jom]

Changes proposed in this Pull Request:

• Updates deprecated usage of Sensei_Utils::user_started_course in courses related code.
• For all usages in Sensei_Course, it was pretty clearly checking for enrolment.
• For the results page status, I used has_started_course. The reason being, if someone has progress and goes to /courses/{slug}/results, they should be able see their actual progress.

Testing instructions:

• Course results page should show proper message if user has ever started course (and hasn't had progress reset when not enrolled).
• When viewing a course you're enrolled in, the course progress meter and message at the top should show up correctly. If you aren't enrolled and there aren't products associated with it, you should see action button to start taking course.
• If your course has preview lessons, when you aren't enrolled you should see button Preview this course.

[yscik]

For the results page status, I used has_started_course. The reason being, if someone has progress and goes to /courses/{slug}/results, they should be able see their actual progress.

The rendering of the lesson list on that page currently checks for enrollment, we should probably make it consistent. This sounds more reasonable, I'll update the other PR.

Otherwise all looks good and works as advertised.