[1.7.x] Fixed #23794 -- Fixed migrations crash when removing a field that's part of index/unique_together.

Backport of 72729f844e0dd9bd01b3874171b89ab0d136a40e from master
[1.7.x] Fixed typo in docs/internals/release-process.txt.

Backport of a3aeba0f9535e59133e49374a6485a85f26d756e from master
[1.7.x] Fixed #23865 -- documented how to assign errors to a field in Model.clean()

Also added a unit test wit the simpler syntax which we have documented,
where the dictionary values are strings.

Backport of 5b26a014a81ba0d404d46e11d2b45c01d92b97e5 from master
[1.7.x] Fixed #23778 -- Added a doc section on using the Django runner for reusable apps.

Backport of 06726965c3e53e9a6b87e1532951a93d5f94f426 from master.
[1.7.x] Fixed typo in forms/models.py comment.

Backport of a7c3d0f288a66d327e71476e9a9bfcc791602b8c from master