make embargo template theme-able

Use "Stanford-style" theming to enable you to put your own embargo
notice up.
missing themed embargo page fails safely to unthemed
Add SHOW_PROGRESS_SUCCESS_BUTTON feature

Show a button at the top of lms/templates/courseware/progress.html if the
lowest nonzero grade cutoff has been reached. Introduce two settings:
PROGRESS_SUCCESS_BUTTON_URL is the href of that button (the course id is
appended); PROGRESS_SUCCESS_BUTTON_TEXT is the text, defaults to
"Download your certificate"
Make the SHOW_PROGRESS_SUCCESS_BUTTON feature more flexible

The button text default is now in the template and i18n-able, a
PROGRESS_SUCCESS_BUTTON_TEXT_OVERRIDE option is offered in settings.
The button url now uses string formatting.
Add test in import code to make sure two overlapping courses aren't imported
Refactored terminology and added loop break for optimization
Fix missing call to translate in shoppingcart model
Fix missing call to translate for shopping cart btn
Switch PDF Textbooks to use portable URLs
install and set up paver before using it

install and set up paver before using it
pass event to callback functions in membership.coffee (LMS-2547)
Fix missing call to translate in receipt page
Update bok-choy page definitions
adding django admin command to change enrollment
fixing example
Wording for batch beta email add with autoenroll (LMS-2551)
grammar
Merge pull request #3366 from IONISx/missing-translations

Missing translations
Merge pull request #3385 from edx/adam/add-event

pass event to callback functions in membership.coffee (LMS-2547)
Added watchdog to Paver requirements