core: make educated guesses for conf name in registration & password reset mail subjects

src/backoffice/tests/auth.py

src/core/locale/de/LC_MESSAGES/django.po

src/core/locale/en/LC_MESSAGES/django.po

...re/templates/registration/core_password_reset_subject.txt

src/core/templates/registration/registration_subject.txt

src/core/views/auth.py

src/plainui/tests/test_auth.py