Fix soft-apply detection of migrations with dependencies
Fixed #21824 -- Added reference to LTS in docs/internals/security.txt
Fixed #21869 -- Fixed docs building with Sphinx 1.2.1.

Thanks tragiclifestories for the report.
Fixed #21870 -- Admin check for list_editable_item

During the admin check for list_editable _check_list_editable_item
should return an empty list if all checks pass. Additionally the
Testcase test_readonly_and_editable was changed to test what the
name implies instead of duplicating the logic of test_readonly.
Fixed typo in error message.
Fixed #21783: More SQLite default fun with nulls.
Unroll choices iterators in field deconstruction
Fixed #21871 -- Fixed Apps.is_installed() for apps with custom label.

Thanks Aymeric for design discussion.
Fixed #21748 -- join promotion for negated AND conditions

Made sure Django treats case .filter(NOT (a AND b)) the same way as
.filter((NOT a OR NOT b)) for join promotion.