[1.7.x] Added a bunch of missing unicode_literals

Refs #22564.
Backport of 12474dace from master.
[1.7.x] 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.
Backport of da9cf53cb from master.
[1.7.x] Updated the messages context processor docs

Refs #20610.
Backport of 7951f125e frmo master.
[1.7.x] Fixed #22586 -- Added LANGUAGE_SESSION_KEY to __all__.

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

Backport of 4492ea5475 from master