apps.core.models.period: Added PeriodManager with:

- filter_is_candidate_or_relatedstudent()
- filter_active()
devilry_student.views.frontpage: Basic active period listing.
devilry_student.views.frontpage: Test only active.
devilry_student.views.frontpage: Usable.
Merge branch 'new-student' of github.com:devilry/devilry-django into new-student