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.