This commit is a rough port to Django 2.1, Python 3 and a Docker based local
development setup. Tests fail/error but migrations and the web frontend are
already runnable. Task queue functionality is untested and translations seem to
have trouble.
- implement osusers.models.SshPublicKey and osusers.models.SshPublicKeyManager
- fix broken osusers.models.tests.test_models
- add new test classes SshPublicKeyManagerTest and SshPublicKeyTest
- add migration for SshPublicKey model