tell automatic form validators to accept empty fields in Course
pin html5 course in home
reorder migrations, because there was 2 0009

Conflicts:
	core/migrations/0013_auto__chg_field_course_publication.py
add new field on course model

Conflicts:
	core/models.py
load thumbnail if there is no video in course intro
test if there is a thumbnail before linking
removes thumbnail from course serializers
Merge branch 'stable'

Conflicts:
	accounts/tests/test_views.py
	core/models.py
	core/serializers.py
	core/templates/course.html
	core/views.py