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
--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
added SITE_* settings
use SITE_HOME to set the home page
added context processor to make site settings available in templates
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