- 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
- use new file tasks to create SFTP and mail base directories for users
- use json serializer as default
- remove TaskResult classes that don't provide any significant benefit