Backport of b47552b4 from master

Thanks Simon Charette for the DeprecationInstanceCheck class.

Backport of 8d1a001e from master

Backport of ae9f9dc3 from master

Thanks Anjul Tyagi for some of the draft text.

Backport of a523d94b from master

This avoids creating an empty list which is invalid HTML 4.

Backport of 53ccffdb from master

Backport of c14b6b52 from master

Backport of 4cd2bdae from master

Backport of a3a6def8 from master

Thanks Tim Graham for the patch series review.
Backport of 50acbf3f from master.

Backport of 4908222a from master.

Backport of ddd6a530 from master.

Backport of d59d3caf from master.

Backport of 6dc7af3e from master.

Backport of d3cfdfb5 from master.

Backport of febd5aee from master.

Backport of 021782d2 from master

Backport of de99f558 from master

Backport of c42123ad from master

Left over Oracle mostly as-is since it's more complicated.

Backport of afe77763 from master

Backport of 53e89ce2 from master

[1.9.x] Fixed #25457 -- Improved formatting of password validation errors in management command output.

Backport of 7372cdeb from master

Backport of a51070e7 from master

Thanks aklim007 for the suggestion.

Backport of 361f6047 from master

Backport of ce531f7a from master

Backport of fd1e54b9 from master

Obsolete since 4a66a692.

Backport of 1f84721f from master

Backport of dfa81bb1 from master

When someone needs to build a custom backend-specific GIS lookup, it
is much easier done if getting the spatial operator class happens in
a dedicated method (no need to rewrite the entire as_sql() method).

Without an explicit 'level', only messages at WARNING or higher
are handled. This makes the config consistent with the docs
which say, "The django catch-all logger sends all messages at
the INFO level or higher to the console."