Revert "Merge pull request #3929 from edx/sarina/lms-2786"

This reverts commit 5c9fe4bb7a6be2e067ac7659edca19bda0f3cbd6, reversing
changes made to 0f2b4cd2b6bdb2ff164e314ff67cfaa3f131f637.
Revert "Merge pull request #3931 from edx/sarina/fixup-external-opaque-keys-impls"

This reverts commit 0f2b4cd2b6bdb2ff164e314ff67cfaa3f131f637, reversing
changes made to b15f3f19514ccdcec152ce21e01fb31f632a5491.

Conflicts:
	cms/djangoapps/contentstore/views/tests/test_item.py
	requirements/edx/github.txt
Revert "Merge pull request #3862 from cpennington/external-opaque-keys"

This reverts commit 81d785d192f46a7399141505951af9a20e838cc1, reversing
changes made to 502b285bc29ed78fc4b93fff2e366741d8dab3aa.

Conflicts:
	requirements/edx/github.txt
Revert "Merge pull request #3900 from edx/dhm/test_has_course"

This reverts commit ca66d982366fd484c08ae239d4938cd34f021a42, reversing
changes made to eeac479c02fef2cf26d5a9cd3bda2ac7296d5c3f.
Revert "Merge pull request #3901 from edx/dhm/opaque-dont-skip"

This reverts commit eeac479c02fef2cf26d5a9cd3bda2ac7296d5c3f, reversing
changes made to 8809c5f0e0916ff04c3a484e8e5c5cbc3ac2879f.
Revert "Merge pull request #3899 from edx/opaque-keys"

This reverts commit 58198b91d4c3fe13c1486061cc6a0296359c0b93, reversing
changes made to eacdc3b6ec59605c6e7390998b9ec515052ec174.
Revert "Merge pull request #3842 from edx/ahodges/documentation/LMS2748"

This reverts commit eacdc3b6ec59605c6e7390998b9ec515052ec174, reversing
changes made to 5ba3acb3cf02989f12cd94931793de6b3e1e89e0.

Conflicts:
	docs/en_us/course_authors/source/change_log.rst