Implement course completion button.

The button is shown on the progress page under the progress graph
when the student's score is above the course criteria.

The url to which the button redirects the user is configured by the
'COURSE_COMPLETION_BUTTON_URL' variable which should be part of the
'EDXAPP_ENV_EXTRA' map.
Remove autoplay for video in course about page

The iframe was generated with autoplay=1, which, sometimes, would cause
the video to play in the background.
Don't hardcode https protocol in email links.

The https protocol was hardcoded in some urls used in emails,
which causes issues for sites that are not configured to use SSL.

Chose the protocol based on the value of request.is_secure() instead.
corrects typo in legacy.py (LMS-1675)