Set up alembic migration usage (#954)

* Set up alembic migration usage

* Fix import order, add isort as alembic hook

* Only run migrations if needed

* Include date as part of migration file name for better sorting

* Skip initial migrations if tables already exist
This eases the transition from a pre-alembic state

* Fix deprecation warning in alembic revision

* Add alembic test stubs

tests/unit_tests/test_alembic.py

@@ -0,0 +1,14 @@
+import pytest
+
+# Test that alembic revisions are applicable and result in the current database
+# See https://github.com/sqlalchemy/alembic/issues/724 for inspiration
+
+
+@pytest.mark.skip("TODO: Implement")
+def test_alembic_revisions_are_applicable():
+    pass
+
+
+@pytest.mark.skip("TODO: Implement")
+def test_alembic_revisions_are_up_to_date():
+    pass