{#
The neutral frame every default HTML mail extends. A message supplies a
`preheader` (the line the inbox shows beside the subject), a `heading`, and
its `content`; everything else - the product name, the footer - is the same
on all of them, so a new e-mail is three short blocks and never a fresh copy
of this file.
Applications brand their mail by shipping their own `_layout.html` (and
optionally `_macros.html`) in the directory `MAIL_TEMPLATE_DIR` points at;
the search order makes theirs win for every message at once.
It is written the way mail has to be written rather than the way web HTML
is: nested tables, fixed 600px width, presentational attributes beside the
styles, and every rule inline. The
{# Shown in the inbox list next to the subject, and nowhere else. The
trailing whitespace stops the client from filling the rest of the
preview line with the top of the message. #}