Fix observer enrollment impacting section privileges of teacher

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

 closes instructuregh-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.

app/models/course.rb

@@ -2935,6 +2935,7 @@ def section_visibilities_for(user, opts = {})
   end
 
   def visibility_limited_to_course_sections?(user, visibilities = section_visibilities_for(user))
+    visibilities = visibilities.select { |v| v[:type] != "ObserverEnrollment"}
     visibilities.all? { |s| s[:limit_privileges_to_course_section] }
   end
 

spec/models/course_spec.rb

@@ -1851,6 +1851,17 @@ def make_date_completed
       expect(@course.course_section_visibility(limited_teacher)).to eq [@section2.id]
     end
 
+    it "correctly limits visibilities for a limited teacher observing a student in different section" do
+      limited_teacher = user_with_pseudonym(name: "Limited Teacher")
+      @course.enroll_teacher(limited_teacher,
+                             limit_privileges_to_course_section: true,
+                             section: @section2)
+      student = user_with_pseudonym
+      @course.enroll_student(student, enrollment_state: "active", section: @section1)
+      add_linked_observer(student, limited_teacher)
+      expect(@course.course_section_visibility(limited_teacher)).to eq [@section1.id, @section2.id]
+    end
+
     it "unlimited teachers can see everything" do
       unlimited_teacher = User.create(name: "Unlimited Teacher")
       @course.enroll_teacher(unlimited_teacher, section: @section2)