restrict admin email to same format as the JSONSchema

restrict admin email to same format as the JSONSchema

Use the same regex as the 'email' format in JSONSchema (copied from there).
Otherwise, users can enter an email address which does not match our
schema and breaks the userlist API Call with a 'return value
verification failed', and one cannot rectify this via the gui
(the user has to manually edit the user.cfg or set the email via
pveum)

i noticed this while testing and using the 'insert emoji' menu in the
context menu

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>