Brought comments in sync with the code in BaseAppCache.
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.
Moving notes for assertNumQueries next to the other changes to test infrastructure
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.
Changed documentation of HttpResponse.content to indicate it's a bytestring
Merge pull request #2067 from dominicrodger/master

Moving notes for assertNumQueries next to the other changes to test infrastructure
Added 1.6.2 release notes stub.
Fixed #21580 -- Clarified why render() does not return a TemplateResponse object.
Fixed #21606 -- Fixed default_lat and default_lon override in OSMWidget
Allowed running GIS form tests that don't require spatial database
Fixed link text for "Testing in Django" in tutorial 5.
Removed usage of to-be-deprecated ADMIN_FOR setting in contributing docs
Fixed #21614 -- improved docs on configuring email for development.
Fixed E127 pep8 warnings.
Clarifed table rewrites in migration docs.

Small nitpicks. All column-related ALTER TABLE commands take an
exclusive table lock in PostgreSQL. The difference is that adding a
column without default doesn't cause a table rewrite, so the lock is
held only for a short time.

The time taken is more accurately proportional to table size, not row
count.
Fixed #21250 -- Made HTTP auth user header configurable in tests

Currently, if the authentication mechanism uses a custom HTTP header
and not REMOTE_USER, it is not easy to test. This commit modifies
remote user tests in order to make them more generic.