Use setuptools entrypoint to register app

Advances and blocks AlekSIS-Core!395 (merged)

pyproject.toml

@@ -62,6 +62,9 @@ flake8-isort = "^4.0.0"
 pytest-cov = "^2.8.1"
 pytest-sugar = "^0.9.2"
 
+[tool.poetry.plugins."aleksis.app"]
+untis = "aleksis.apps.untis.apps:UntisConfig"
+
 [tool.black]
 line-length = 100
 exclude = "/migrations/"