feat(settings): use configured site domain for undefined email addresses

Previously "localhost" was used as the default email domain.
Now the configured site domain is used instead.
3 jobs for ci-tests in 2 minutes and 58 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #5857
make_deb_package

00:02:06

 
  Test
passed #5859
deb_package_test

00:00:51

passed #5858
make_test

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