Improve vacation notice appearance

I noticed that vacation notices generated by James look somewhat strange in Thunderbird, and probably other mail clients as well: It shows both the HTML and plain text variant of the message (see attached screenshot). This is likely caused by the Content-Type of multipart/mixed.

I suggest we change this to multipart/alternative instead, so a HTML capable client will only show the HTML part, while a text-only client will show the text part instead.

I also noticed that, after setting an HTML message and an empty text message by accident, the generated text part is naturally empty. This is OK for HTML-capable clients, but in the worst case a text-only client would show just an empty mail, even though there is content in the HTML part.

I suggest we make the vacation notice generation more robust, and also generate a text message from HTML in this case, which will be more useful to the recipients.