Initial set of backend API for event webhook notifications for the following events:
* Crawl started (including boolean indicating if crawl was scheduled)
* Crawl finished
* Upload finished
* Archived item added to collection
* Archived item removed from collection
Configuration of URLs is done via /api/orgs/<oid>/event-webhook-urls. If a URL is configured for a given event, a webhook notification is added to the database and then attempted to be sent (up to a total of 5 tries per overall attempt, with an increasing backoff between, implemented via use of the backoff library, which supports async).
webhook status available via /api/orgs/<oid>/webhooks
(Additional testing + potential fastapi integration left in separate follow-ups
Fixes#1041
