_includes/public-inbox.html: Use "Re:" over "RE:" for mailto link...From RFC 5322 [0]:
> When used in a reply, the field body MAY start with the
> string "Re: " (an abbreviation of the Latin "in re", meaning "in the
> matter of") followed by the contents of the "Subject:" field body of
> the original message.
[0]: https://www.rfc-editor.org/rfc/rfc5322