Improved strip_tags and clarified documentation

The fact that strip_tags cannot guarantee to really strip all
non-safe HTML content was not clear enough. Also see:
https://www.djangoproject.com/weblog/2014/mar/22/strip-tags-advisory/
Avoided transactional DDL on castrated databases.

Fixed a test failure that appeared after 753a22a6, although the bug
existed before that commit.

Refs #22308.
Added release note about strip_tags improvement
Made ModelForms raise ImproperlyConfigured if the list of fields is not specified.

Also applies to modelform(set)_factory and generic model views.

refs #19733.
Avoided transactional truncates on amateur databases.

Fixed a test failure that appeared after 753a22a6, although the bug
existed before that commit.

Refs #22308.
Tweaked strip_tags tests to pass on Python 3.3
Fixed #22294 -- Prevented converting length filter output to string

Thanks Steve Pike for the report.
Fixed #22256 -- Replaced bad fallback for missing PATH

Thanks Baptiste Mispelon for the review.
Fixed #22241 -- Selectively added line breaks in admin read-only fields

When content is supposed to contain HTML, we do not try to add
line breaks in read-only contents.
Thanks Alexander Todorov for the report.
Fixed a failing test introduced in 918a16bc4c099ab0cae72a231de3e99e2a9d02cb.

Refs #22307.
Removed an unused variable.
Fixed #17922 -- Added required_css_class to form label.