Admin unique email constraint is not present in admin form

[techi602]

Administration form for editing administrator does not have constraint for unique email. Therefore submitting existing administrator e-mail into the form results in error 500 (SQL unique index error).

To Reproduce

In administration go to Administrators --> Administrators overview. Create two administrators with unique e-mail. Then update 2nd administrator and set email from 1st administrator and submit the form.

Additional context

The validation for unique email is already implemented in customer update form.

https://github.com/shopsys/shopsys/blob/master/packages/framework/src/Form/Admin/Administrator/AdministratorFormType.php#L66-L72
vs.
https://github.com/shopsys/shopsys/blob/master/packages/framework/src/Form/Admin/Customer/User/CustomerUserFormType.php#L151