Prevent admin from creating users with empty username

[coalio]

A error message shows when the admin tries to submit the form if the validation fails.

The error message was translated to Czech, Deutsch, English, Spanish and French, which are the currently compatible languages in the website.

[Wingysam]

Thanks! Could you please put the validation logic in the server-side route handler instead of on the client please?

[coalio]

@Wingysam done!

[Wingysam]

Perfect, thank you!