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
professor messaging infra structure
adding course to ProfessorMessage
viewsets for professor message
missing send_mail import
Merge branch 'master' into professormessages
adding validation to professor message serializers
Merge branch 'master' into professormessages
Merge branch 'master' into professormessages
email templates no banco
fix #306
Merge branch 'master' into professormessages
link lessons admin in courses list, fixes #308
html5 course as home