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
link lessons admin in courses list, fixes #308
html5 course as home
--use-mirrors has been deprecated
added spanish, italian and english as supported languages in settings
Updated tranlations strings in templates, refs #156
Merge pull request #318 from barraponto/no-use-mirrors

pip's `--use-mirrors` has been deprecated
adding locales it and es
fix build
clear comented code
fix notes count in my notes
mark units as done when there is no activity, fixes #320
min-width label on new course
remove add professors from admin course header