Confirmation email format

The body of confirmation emails is always HTML currently. It would be ideal to have a toggle to select text or HTML, as HTML is not necessarily needed in all cases.

At a minimum, it should at least be documented that the body is HTML in both the help text and the docs. Since this is HTML, newlines have no effect and `
` tags must be used. This is a point of confusion among users.

Start by locating the confirmation email generation, its help text, and the relevant documentation. Determine whether the issue should provide a text/HTML toggle or only document the current HTML behavior; done means the selected format works correctly, or the HTML requirement and
newline behavior are clearly documented.

索引モデルが issue の本文から書いたものです。