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

bootstrap.sh

@@ -12,5 +12,10 @@ python manage.py generate_secret_key
 # Ensure the DB folder is owned by the right user
 chown -R www-data: /etc/linkding/data
 
+# Start background task processor using supervisord, unless explicitly disabled
+if [ "$LD_DISABLE_BACKGROUND_TASKS" != "True" ]; then
+  supervisord -c supervisord.conf
+fi
+
 # Start uwsgi server
 uwsgi uwsgi.ini