Responses to Adam's comments; reset common.py, i18n compliance, deleted extraneous email.html file, fixed an HttpResponse, deleted unnecessary commented-out code, some small style tweaks

set max enrollment for downloads to 200

Add EmptyDataRawDescriptor.

Valera/carry over of add option transcripts remove onhover 2

Course Modes Expiration

There were previous merge conflicts in the file.

When auto-show/auto-hide is disabled, the captions will be shown when
the user will move the mouse over the "CC" button. They will then
auto-hide after a while if the user doesn't continue to move the
mouse over the "CC" button, or over the captions themselves, or use
the keyboard to select a specific caption. If the mouse pointer is not
over the captions or the "CC" button, they will hide after a while.

This is the key in the fix for bug BLD-355: Transcript hovers over
over videoplayer itself for PKU videos.

Front-end functionality was not removed. When flag is set to "True", old
behaviour of autohiding of controls and captions will be enabled.