Creating a technical user should require a subject

[ghost]

This is a direct continuation of the work for #2088, which has been merged to development in the meantime.
After discussing today with @erlemantos , I realized the persistence of a string representing the subject should actually take place when creating a technical user.
The only logical point where an administrator creates a technical user is when they are adding a (new) user to a namespace.
Therefore, the modal I created in #2088 needs to be completed with a required text field stating the "subject".
That field will require validation both on the front-end and back-end - will need to figure out what exactly constitutes a valid string representing a subject.
Persisting the additional field should be trivial as the table design already has it and it'll just be about writing trivial logic at controller / back-end level.

[ghost]

PR here.

[ghost]

Note on subject validation: at the moment I'm just using a 4+ alnum rule.
This can be fine-tuned relatively easily in the future, although it has to be done in both front-end and back-end.

[ghost]

#2209 merged. Closing.