SQLAlchemy autoflush warnings

[FrostyX]

From our testsuite:

tests/test_mail.py::TestMail::test_outdated_chroot_message
  /home/jkadlcik/git/copr/frontend/coprs_frontend/coprs/models.py:495: SAWarning: Object of type <MockChroot> not in session, add operation along 'DistGitBranch.chroots' will not proceed (This warning originated from the Session 'autoflus
h' process, which was invoked automatically in response to a user-initiated operation.)
    return self.group.at_name if self.is_a_group_project else self.user.name