[1.6.x] Fixed #22998 -- Updated the fast_delete logic for GFKs

Backport of 6e2b82fdf6 from master
[1.6.x] PEP8 cleanup

Backport of f8df55050c from master
[1.6.x] Added Chrome/IE support for a selenium test.

Backport of 5954aa6db0 from master plus additional changes...
[1.6.x] Fixed #23023 -- Added warning against local-memory cache.

Thanks django at kerz.id.au.

Backport of 7d0519c725 from master
[1.6.x] Fixed reference to pk in RedirectView.get_redirect_url doc.

Backport of d9f78d5344 from master
[1.6.x] Added 1.6.6 release notes for #22998.