fix(mail): send internal link

[ArtificialOwl]

generate an internal link instead of public link when sending mail to local account

[ArtificialOwl]

/backport to stable33

[ArtificialOwl]

/backport to stable32

[T0mWz]

@ArtificialOwl , I wonder if this change is sufficient, as I think that still a public link token is generated, although this is not actively communicated to the user. https://github.com/nextcloud/circles/blob/master/lib/ShareByCircleProvider.php#L145

This change does not, in fact, resolve the underlying problem. Namely that a shareToken is generated, but it is only not mailed. This means the vulnerability remains, allowing unauthorised access to the data if the correct URL is guessed

-		$share->setToken($this->token(15));
+		$share->setToken('');