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/tests/test_signals.py

@@ -0,0 +1,15 @@
+from unittest.mock import patch
+
+from django.test import TestCase
+
+from bookmarks.services import tasks
+from bookmarks.tests.helpers import BookmarkFactoryMixin
+
+
+class SignalsTestCase(TestCase, BookmarkFactoryMixin):
+    def test_login_should_schedule_snapshot_creation(self):
+        user = self.get_or_create_test_user()
+
+        with patch.object(tasks, 'schedule_bookmarks_without_snapshots') as mock_schedule_bookmarks_without_snapshots:
+            self.client.force_login(user)
+            mock_schedule_bookmarks_without_snapshots.assert_called_once_with(user.id)