Implement technical user creation quota

[ghost]

Following up on #2503 , the most important development would be to implement a (e.g. 2 technical users) quota for regular users.

With the "created_by" field, we now know who creates technical users (not retroactively of course), so we have better tooling to stop abuse (i.e. unsupervised creation of large numbers of technical users with total "impunity").

This next step would allow us to automate the verification and prevent it before it happens.