Fix observer enrollment impacting section privileges of teacher

[cgravatt]

visibility_limited_to_course_sections? should return true if all of the enrollments have limited_section_privileges ignoring observer enrollments

closes gh-2245

Test Plan:

• Enroll a teacher in a section of course with limit section privileges set to true
• Enroll a student in a different section of the same course with limit section privileges set to true
• Set up the teacher as an observer of the student (for example the teacher teaches a section of a course but has a son or daughter taking a different section of the same course) Confirm the teacher cannot view all sections and can only see the two sections.

[cgravatt]

Any feedback on this?

[cgravatt]

Any update on this?