Skip email domain check when admin users adds user manually

[Zettat123]

Fix #27457

Administrators should be able to manually create any user even if the user's email address is not in EMAIL_DOMAIN_ALLOWLIST.

[GiteaBot]

I was unable to create a backport for 1.21. @Zettat123, please send one manually. 🍵

go run ./contrib/backport 29522
...  // fix git conflicts if any
go run ./contrib/backport --continue

[Zettat123]

In v1.21 the ValidateUser also calls ValidateEmail. Maybe we need to change more code if we need to backport

gitea/models/user/user.go

Lines 802 to 817 in 4ef7e49

	// ValidateUser check if user is valid to insert / update into database
	func ValidateUser(u *User, cols ...string) error {
	if len(cols) == 0 || util.SliceContainsString(cols, "visibility", true) {
	if !setting.Service.AllowedUserVisibilityModesSlice.IsAllowedVisibility(u.Visibility) && !u.IsOrganization() {
	return fmt.Errorf("visibility Mode not allowed: %s", u.Visibility.String())
	}
	}
	if len(cols) == 0 || util.SliceContainsString(cols, "email", true) {
	u.Email = strings.ToLower(u.Email)
	if err := ValidateEmail(u.Email); err != nil {
	return err
	}
	}
	return nil
	}

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UPDATE: It's difficult to modify ValidateUser so this PR won't be backported