requirements.txt: updated celery package version requirements
website.celery: create celery app
website.admin: do not unregister the djcelery models
website.settings: added some new celery settings and updated others. Added 'celery.tasks' to our logging config which resolved the missing celery log messages issue
website.async_tasks: removed redundant abstract task classes. Removed the settings that configured retries since do not handle retries in the tasks
stopcelery: force-kill celery if it doesn't stop after asking nicely
Changes: