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.
No longer chunk queries when sending out bulk email (LMS-2718)

refactor _generate_items_for_subtask not to chunk queries

use django's iterator methor for querysets

remove bulk email query settings

use read_replica if available, since this is read-only

update changelog
Merge pull request #3841 from edx/fix/large-email-queries-2

Fix/large email queries 2
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.
Delete orphans was trying to get non-existent 'draft' modulestore