* also, add a new config item `THEME_TUW_TEST_INSTANCE_URL` that will be
  used in the "final note" block in the email template
* if that value is set to `None`, then the block will be skipped
  entirely
* also, mock view function from Invenio-Config-TUW to not depend on it
  for tests

* also, remove unused utility function

* replace the network request to doi.org

* use more modern test fixtures that take care of initializing the
  app and extensions automatically
* throw out some tests

* also remove Flask dependency, because that's already covered