Added a bunch of missing unicode_literals

Refs #22564.
Fixed #22564 -- Prevented unneeded bytestrings in migrations

In some cases, this could lead to migrations written with Python 2
being incompatible with Python 3.
Thanks Tim Graham for the report and Loïc Bistuer for the advices.
Updated the messages context processor docs

Refs #20610.
Fixed #22586 -- Added LANGUAGE_SESSION_KEY to __all__.

Thanks david.fischer.ch at gmail.com for the report.