API: creating last project before limit returns 404

When the last project is created before the limit is reached, the API (`POST /projects`) returns a 404 error instead of status code 201 (Created). The project itself gets created and is available in the database.

A similar behaviour can be observed via web client when the user creates the last project, he receives a notification that the limit is reached. As before the project itself gets created. I will open another PR for this.

Tests are added to check the behaviour and the fix solves the issue for the API.