[1.7.x] 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.

5ea34f3f86 from master.
[1.7.x] Fixed #22476: Couldn't alter attributes on M2Ms with through= set
[1.7.x] Removed unused imports in makemigrations

Backport of fc32e9c0d from master.
[1.7.x] Appeased flake8.

Backport of b1432bfc from master
[1.7.x] Reorganized backends tests.

This reduces the number of explicit vendor checks.

Backport of d6672631 from master
[1.7.x] Simplified indexes tests.

Backport of 43a80f48 from master
[1.7.x] Replaced vendor checks by three feature flags.

Backport of c70a61eb from master
[1.7.x] Split ignores_nulls_in_unique_constraints feature.

Oracle and SQL Server don't have exactly the same limitations. It's
worth treating them differently.

Backport of cff59bed from master
[1.7.x] Added feature flags for introspection capabilities.

Backport of 99d9fa32 from master