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.
Updating ORA2 requirements for patch fix
Explicitly declare `tags` in dog stats increment calls
Fix source editor for the case of no CodeMirror editor.

In the HTML component, there is no longer a tabbed CodeMirror Editor. STUD-1516.
Use the correct response when using login.
Merge pull request #3253 from edx/sarina/fix-capa-calls

Explicitly declare `tags` in dog stats increment calls
Merge pull request #3244 from edx/christina/latex-bug-release

Fix source editor for the case of no CodeMirror editor.
Merge pull request #3256 from edx/diana/fix-grading-service-login

Return correct response when using _try_with_login
Revert "updated pdf viewer"

This reverts commit b0a7443570d929e705c295c1511f3188f4e41f69.
Revert refund instruction message
Merge pull request #3265 from edx/flowerhack/refund-message-patch

Revert refund instruction message
Refactor speed control.
Add fixes.
Update docstrings.
Removed unused helpers.
Fixed import of "dog_stats_api" in staff_grading_service.py.
LMS-2521
Merge pull request #3279 from edx/waheed/lms2521-staff-grading-panel-broken-fixed

Fixed import of "dog_stats_api" in staff_grading_service.py.