Fix password-manager username not saved on customer account creation (#6043)

* Fix password-manager username not saved on customer account creation

* Fix tests/make email not required

src/pretix/presale/forms/customer.py

@@ -296,7 +296,8 @@ class SetPasswordForm(forms.Form):
     }
     email = forms.EmailField(
         label=_('Email'),
-        disabled=True
+        widget=forms.EmailInput(attrs={'autocomplete': 'username', 'readonly': 'readonly'}),
+        required=False,
     )
     password = forms.CharField(
         label=_('Password'),