Fixed #21473 -- Limited language preservation to logout

Current language is no longer saved to session by LocaleMiddleware
on  every response (the behavior introduced in #14825).
Instead language stored in session is reintroduced into new session
after logout.

Forward port of c558a43fd6 to master.
Fixed #21591 -- Added documentation about contrib.messages.get_messages.

Refs https://code.djangoproject.com/ticket/21591
Thanks to track user merb for the report.
Improved translation maintenance script

Do not fetch 'en' catalogs, neither files with translation stats
lower than 5%. Centralized resource filtering.
Fixed PEP257 violations (single-quoted docstrings) in utils.text.
Fixed #21574 -- Handle bytes consistently in utils.text.normalize_newlines.

All input is now coerced to text before being normalized.
This changes nothing under Python 2 but it allows bytes
to be passed to the function without a TypeError under Python3
(bytes are assumed to be utf-8 encoded text).

Thanks to trac user vajrasky for the report.
Copied 1.6.1 release notes from stable/1.6.x branch.
Fixed #5418 -- added assertNoBrokenLinks to test system

Check for broken links in a rendered page. By default only internal links are
checked, if `only_internal` is `False`, external links are checked too. By
default, redirections are considered valid and not followed unless specified
with `follow=True`.

This commit adds the patch, the tests and the docs where the possible side
effects of this test are noted.