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

siteroot/settings/dev.py

@@ -43,6 +43,11 @@ LOGGING = {
         'django.db.backends': {
             'level': 'ERROR',  # Set to DEBUG to log all SQL calls
             'handlers': ['console'],
+        },
+        'bookmarks.services.tasks': {  # Log task output
+            'level': 'DEBUG',
+            'handlers': ['console'],
+            'propagate': False,
         }
     }
 }