Open-event-server: Disallow creating users with same email.

[dr0pdb]

Is your feature request related to a problem? Please describe.

Right now we can create users with the same email. If we soft-delete a user account and try to create an account with the same email again, the server allows creating it but sends HTTP 500 when doing so since it voilates the unique constraint in the db.

[softdev07]

@srv-twry can I take this?

[dr0pdb]

@mishuvs could you please do this quickly. Need this. Moreover it's a simple bug so shouldn't take long.