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/base.py

@@ -42,6 +42,7 @@ INSTALLED_APPS = [
     'django_generate_secret_key',
     'rest_framework',
     'rest_framework.authtoken',
+    'background_task',
 ]
 
 MIDDLEWARE = [
@@ -166,3 +167,14 @@ ALLOW_REGISTRATION = False
 
 # URL validation flag
 LD_DISABLE_URL_VALIDATION = os.getenv('LD_DISABLE_URL_VALIDATION', False) in (True, 'True', '1')
+
+# Background task enabled setting
+LD_DISABLE_BACKGROUND_TASKS = os.getenv('LD_DISABLE_BACKGROUND_TASKS', False) in (True, 'True', '1')
+
+# django-background-tasks
+MAX_ATTEMPTS = 5
+# How many tasks will run in parallel
+# We want to keep this low to prevent SQLite lock errors and in general not to consume too much resources on smaller
+# specced systems like Raspberries. Should be OK as tasks are not time critical.
+BACKGROUND_TASK_RUN_ASYNC = True
+BACKGROUND_TASK_ASYNC_THREADS = 2