Fixed #20874 -- bump_prefix() in nested subqueries

Also made some cleanup to build_filter() code by introducing submethods
solve_lookup_type() and prepare_lookup_value().
Fixed overflow for the "Recent Actions" widget on the admin index.

Previously the CSS targeted "li.changelink" and therefore didn't
work for the "add" and "delete" actions.

Refs #14868.
Fixed #20883 -- Made model inheritance find parent links in abstract parents
Merge pull request #1470 from loic/admin.actionlist

Fixed overflow for the "Recent Actions" widget on the admin index.
Fixed test failure on MySQL

The fix for #20874 caused a MySQL specific failure.
Fixed is_safe_url() to reject URLs that use a scheme other than HTTP/S.

This is a security fix; disclosure to follow shortly.
Apply autoescaping to AdminURLFieldWidget.

This is a security fix; disclosure to follow shortly.
Added 1.4.6/1.5.2 release notes.
Fixed #20555 -- Make subwidget id attribute available

In `BoundField.__iter__`, the widget's id attribute is now passed to
each subwidget. A new id_for_label property was added to ChoiceInput.
Removed unneeded conditional_escapes from the testsuite.
Fixed #20864 -- Made the test client use common method for performing requests.
Fixed #11400 -- Passed kwargs from AbstractUser.email_user() to send_mail()

Thanks Jug_ for suggestion, john_scott for the initial patch,
and Tim Graham for code review.
Fixed #20709 -- Allowed {% widthratio %} to accept an "as" parameter.

Thanks clay.evil@ for the suggestion.
Added some doc links for django.contrib.messages