Moved ServerHandler helper class to tests.

It has been only used in the builtin_servers tests since Django 1.4.
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.
Fixed #22476: Couldn't alter attributes on M2Ms with through= set
Removed unused imports in makemigrations
Appeased flake8.
Reorganized backends tests.

This reduces the number of explicit vendor checks.
Simplified indexes tests.
Replaced vendor checks by three feature flags.
Split ignores_nulls_in_unique_constraints feature.

Oracle and SQL Server don't have exactly the same limitations. It's
worth treating them differently.
Added feature flags for introspection capabilities.
Fixed #22588 -- Fix RegexValidator __eq__

Compare parameters instead of re.pattern instances, and add the other
parameters to the comparison. Also add a __ne__ to make assertNotEqual
work properly.
Fixed #22563: Ignore AUTH_USER_MODEL errors in from_state
Fixed #22325: Ignore __first__ dependencies to your own app