Fixed #26679 -- Documented which methods are disabled on RelatedManager if there's an intermediate table.

Made it consistently read the PO file, decode its contents and then
check for the non-breaking space Unicode code point.

Previously we were erroneously skipping the interpretation of what we
read as UTF-8 text.

This was causing the test to fail on Windows with Python 3.5.

`subprocess.Popen` doesn't accept enviroment vars with Unicode var name
or value.

This behavior was removed in 67d98441
but is needed to prevent a crash in formtools.

Thanks Tim for the review.

This was reflected by 27 i18n test errors (not failures) on such
platform caused by corrupt intermediate PO catalog files fed to
`msgmerge(1)`.

Made it handle in-memory PO/POT file contents with normalized line
separators (`'\n'`).

We need to perform manually this after using `subprocess.Popen` as we
can't pass it `universal_newlines=True`.

Also fixes #26670 in the process as this commit refactors the same code
section.

Thanks Jasper Maes for the detailed report.

Broke the initial migration in two to work around #25530 and added
'django.contrib.auth' to the available_apps to make sure its tables are also
flushed as Oracle doesn't implement cascade deletion in sql_flush().

Thanks Tim for the report.

Mistakenly pushed to django/django instead of another repo

This reverts commit 6dde884c.