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/management/commands/clean_tasks.py

@@ -0,0 +1,15 @@
+from background_task.models import Task, CompletedTask
+from django.core.management.base import BaseCommand
+
+
+class Command(BaseCommand):
+    help = "Remove task locks and clear completed task history"
+
+    def handle(self, *args, **options):
+        # Remove task locks
+        # If the background task processor exited while executing tasks, these tasks would still be marked as locked,
+        # even though no process is working on them, and would prevent the task processor from picking the next task in
+        # the queue
+        Task.objects.all().update(locked_by=None, locked_at=None)
+        # Clear task history to prevent them from bloating the DB
+        CompletedTask.objects.all().delete()