0.4 LPO performance update - Fix learner-metrics endpoint

Improved performance and fixed filter for the 'learner-metrics' API endpoint

Updated figures.admin
- Added 'UserRelatedMixin' to add a search bar on user fields ( username, email, profile__name) and abstracted the 'user_link' method. Added this mixin to the LearnerCourseGradeMetrics and EnrollmentData model views
- Added more filter terms to both LearnerCourseGradeMetrics and EnrollmentData model views
Added new module, figures.queries
- This abstracts the queryset contruction from the viewset and makes testing the queryset easier in the Django shell without needing to invoke the viewset. Probably a pattern I'll continue
- See the 'site_users_enrollment_data' function docstring for details
Cleaned up figures.serializers
- Minor stuff: Removed list_serializer_class from LearnerMetricsSerializerV2 and removed commented out code
Reworked figures.views.LearnerMetricsViewSetV2
- Removed the query to get course ids. Now just relies on the 'course' query params for course ids. The query will filter out invalid 'course' values. See notes above on 'figures.query'. This helps reduce execution time
- Added new pattern to figures.views by calling an external function to construct the queryset to help with testing and debugging

The Hawthorn tests were not running due to a recent typo I added in the tox.ini 'commands' section. Fixed that.
Also calling out Hawthorn in the same way as Ginko and Juniper. Now all have Open edX release specific pytest-.ini files and Hawthorn now specifies the 'OPENEDX_RELEASE' also
This means that regardless of what the default environment that is set in the settings, the Hawthorn tests will specify the proper environment