Change behavior of existing v0.5 mobile api /api/mobile/v0.5/users/{username}/course_enrollments/ to exclude expired courses due to audit duration limits from the result set.
Add v1 version of mobile api /api/mobile/v1/users/{username}/course_enrollments/ that returns all enrollments and includes a field audiit_access_expires that contains expiration date (or null).
REV-495
Previously, the mobile api did not check for pre-requisite courses
or entrance exams. This change checks for these milestones and then
returns course content accordingly.
Discovered a case I missed on the original version of this. See
https://github.com/edx/edx-platform/pull/5798
JIRA: MA-76
Also fixes checking for mobile_access when course is None
JIRA: MOB-978
