AA-152: Course Start Date for PLS

Updated the displayed start date for a learner in a self paced course
that appears in the Learner Dashboard and the Dates tab.
Used the max of the course start date and the learner's enrollment date.

lms/djangoapps/courseware/date_summary.py

@@ -248,7 +248,11 @@ class CourseStartDate(DateSummary):
 
     @property
     def date(self):
-        return self.course.start
+        if not self.course.self_paced:
+            return self.course.start
+        else:
+            enrollment = CourseEnrollment.get_enrollment(self.user, self.course_id)
+            return max(enrollment.created, self.course.start) if enrollment else self.course.start
 
     @property
     def date_type(self):