Use setuptools entrypoint to register app

Advances and blocks AlekSIS-Core!395 (merged)

pyproject.toml

@@ -59,6 +59,9 @@ flake8-isort = "^4.0.0"
 pytest-cov = "^2.8.1"
 pytest-sugar = "^0.9.2"
 
+[tool.poetry.plugins."aleksis.app"]
+chronos = "aleksis.apps.chronos.apps:ChronosConfig"
+
 [tool.black]
 line-length = 100
 exclude = "/migrations/"