When the project limit is reached the user is not allowed to create new ones.
Instead of error code 404 the status code 403 (Forbidden) is returned with error
message via API.

The previous call `saved?` is restored in the `POST /projects` method in the API.
It is refactored to check if the record is persisted. This is useful to not validate
the record again after saving. This fixes the returned status code in the web client
too. If the last project is created via web client instead of error notification
the project page is shown.

A new queue of "project_web_hook" is used to process web hooks asynchronously, allowing each to succeed/fail (and be retried) independently.

(Basically, project web hooks now process the same as system hooks.)