Don't hardcode https protocol in enrollment emails.

Use http or https, depending on the value of `request.is_secure()`.
That is how links in most other emails are handled.
Move to OpaqueKey implementations from the external library

[LMS-2757]
Fix the title of edit modals from the unit page.
show a friendly message for an empty course rather than an error page
Fix location.url [LMS-2786]
Merge pull request #3931 from edx/sarina/fixup-external-opaque-keys-impls

Use external OpaqueKey implementations
Merge pull request #3929 from edx/sarina/lms-2786

Fix location.url [LMS-2786]
Merge pull request #3754 from theJohnnyBrown/empty-course-error

show a friendly message for an empty course rather than an error page
Add span tag.
Merge pull request #3917 from edx/oleg/add-span-tag-matlab

Accept span tag from xqueue. BLD-1006.
Merge pull request #3196 from mtyaka/enrollment-emails-http

Don't hardcode https protocol in enrollment emails.
Merge pull request #3933 from edx/andya/edit-container-fixes

Fix the title of edit modals from the unit page.
removes qualaroo from studio (STUD-1684)