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

requirements.txt

@@ -39,10 +39,10 @@ django-sass-processor==1.4
     # via -r requirements.in
 django-widget-tweaks==1.5.0
     # via -r requirements.in
-django4-background-tasks==1.2.9
-    # via -r requirements.in
 djangorestframework==3.14.0
     # via -r requirements.in
+huey==2.5.0
+    # via -r requirements.in
 idna==3.6
     # via requests
 josepy==1.14.0
@@ -69,7 +69,6 @@ requests==2.31.0
 six==1.16.0
     # via
     #   bleach
-    #   django4-background-tasks
     #   python-dateutil
 soupsieve==2.5
     # via beautifulsoup4