fix(plainui+backoffice): improve handling of invalid reset tokens

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

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

src/backoffice/templates/registration/reset_password.html

src/backoffice/tests.py

src/backoffice/urls.py

src/backoffice/views/auth.py

src/plainui/jinja2/plainui/password_reset_form.html

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

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

src/plainui/tests.py

src/plainui/urls.py

src/plainui/views.py