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/migrations/0009_bookmark_web_archive_snapshot_url.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.20 on 2021-05-16 14:35
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookmarks', '0008_userprofile_bookmark_date_display'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='bookmark',
+            name='web_archive_snapshot_url',
+            field=models.CharField(blank=True, max_length=2048),
+        ),
+    ]