Summary:
Adds a new endpoint to update webhook.
Perform some refactoring to allow code reuse from endpoint allowing to _subscribe and _unsubscribe webhooks.
One aspect of webhook is that url are stored in the home db while the rest of the fields (tableRef, isReadyColRef, ...) are stored in sqlite. So care must be taken when updating fields, to properly rollback if anything should fail.
Follow up diff will bring UI to edit webhook list
Test Plan: Updated doc api server tests
Reviewers: jarek
Reviewed By: jarek
Subscribers: paulfitz
Differential Revision: https://phab.getgrist.com/D3821
