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.
Bump DatabaseCreation deprecation to 2.0 (refs #22340)
Fixed #22337: FileSystemStorage marked as deconstructible and tested.
Fixed #22576: Ensure makemigrations doesn't touch the database.
Improve docs around deconstruction/serialisation (refs #22337)
Forward ported docs of security issues from df81625da6.
Updated some docs for the delayed deprecation of legacy table creation; refs #22340.
Insert space.
Fixed #22563: Better error message when trying to change AUTH_USER_MODEL

You're not allowed to do this after you've made migrations; see ticket
for more details.
Fixed #22581: Pass default values for schema through get_db_prep_save()
Fixed #22496: Data migrations get transactions again!
Merge pull request #2639 from alimony/patch-1

Insert space
Restored site header text in password reset view.

It's the one shown when the optional integration described in
https://docs.djangoproject.com/en/dev/ref/contrib/admin/#adding-a-password-reset-feature
is used.

Follow-up to commits 6f470650d0 and 1d42a86ec7, together they fix
different small UI regressions after a962286b74.

Refs #21293.