Replace django-background-tasks with huey (#657)

* Replace django-background-tasks with huey

* Add command for migrating tasks

* Add custom admin view

* fix dockerfile

* fix tests

* fix tests in CI

* fix task tests

* implement retries

* improve config

* workaround to avoid running singlefile tasks in parallel

* properly kill single-file sub-processes

* use period task for HTML snapshots

* clear task lock when starting task consumer

* remove obsolete cleanup task command

siteroot/settings/prod.py

@@ -49,7 +49,12 @@ LOGGING = {
             "level": "INFO",
             "handlers": ["console"],
             "propagate": False,
-        }
+        },
+        "huey": {
+            "level": "INFO",
+            "handlers": ["console"],
+            "propagate": False,
+        },
     },
 }