API: tests to show incorrect behavior when reaching project limit

When reaching the project limit the API returns an error code 404 on the last possible
project. The project itself is created and is available in the database (seems valid).
A similar behavior can be observed when reaching the project limit via web client, but
in this case the user is notified that the maximum number of projects is reached. The project
itself is still created and can be accessed.

Tests are added to check the behavior when reaching the project limit or one tries
to exceed it via the API.
API: fixes return code when creating last project before reaching limit

When creating the last project via API when reaching the project limit a status code
of 404 (Not found) is returned instead of 201 (Created). The fix checks now correctly if
the project could be saved.
API: refactored last fix, project limit in web client is fixed too

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.
Merge pull request #2988 from Asquera/error_project_limit

API: creating last project before limit returns 404