Fixed #29695 -- Added system checks for admin's app dependencies and TEMPLATES setting.

[math-a3k]

• Add the following to AdminSite checks:

• admin.E403: A DjangoTemplates instance must be configured in TEMPLATES
• admin.E404: context_processors.messages must be enabled in DjangoTemplates
• admin.E405: django.contrib.auth must be in INSTALLED_APPS
• admin.E406: django.contrib.messages must be in INSTALLED_APPS
• admin.E407: django.contrib.sessions must be in INSTALLED_APPS
• admin.E408: AuthenticationMiddleware must be in MIDDLEWARE
• admin.E409: MessageMiddleware must be in MIDDLEWARE.

• Improve the error message of admin.E402 (auth context processor needed in Django
  Templates if default auth backed is used).

• Updated the documentation and tests accordingly.

As a side-effect, two tests needed tweaking:

• migrations.test_commands.tests.test_migrate_with_system_checks
  Left a note for the maintainer because admin checks are triggered without the
  admin app installed

• admin_scripts.tests.ManageCheck.test_complex_app.

This is motivated by #9682