Create snapshots on web.archive.org for bookmarks (#150)

* Implement initial background tasks concept

* fix property reference

* update requirements.txt

* simplify bookmark null check

* improve web archive url display

* add background tasks test

* add basic supervisor setup

* schedule missing snapshot creation on login

* remove task locks and clear task history before starting background task processor

* batch create snapshots after import

* fix script reference in supervisord.conf

* add option to disable background tasks

* restructure feature overview

bookmarks/signals.py

@@ -0,0 +1,8 @@
+from django.contrib.auth import user_logged_in
+from django.dispatch import receiver
+from bookmarks.services import tasks
+
+
+@receiver(user_logged_in)
+def user_logged_in(sender, request, user, **kwargs):
+    tasks.schedule_bookmarks_without_snapshots(user.id)