nmbug-status: Add Page and HtmlPage for modular rendering

nmbug-status: Add Page and HtmlPage for modular rendering

I was having trouble understanding the logic of the longish print_view
function, so I refactored the output generation into modular bits.
The basic text rendering is handled by Page, which has enough hooks
that HtmlPage can borrow the logic and slot-in HTML generators.

By modularizing the logic it should also be easier to build other
renderers if folks want to customize the layout for other projects.

Timezones
=========

This commit has not effect on the output, except that some dates have
been converted from the sender's timezone to UTC due to:

  -            val = m.get_header(header)
  -            ...
  -            if header == 'date':
  -                val = str.join(' ', val.split(None)[1:4])
  -                val = str(datetime.datetime.strptime(val, '%d %b %Y').date())
  ...
  +                value = str(datetime.datetime.utcfromtimestamp(
  +                    message.get_date()).date())

I also tweaked the HTML header date to be utcnow instead of the local
now() to make all times independent of the generator's local time.
This matches Gmane, which converts all Date headers to UTC (although
they use a 'GMT' suffix).  Notmuch uses
g_mime_utils_header_decode_date to calculate the UTC timestamps, but
uses a NULL tz_offset which drops the information we'd need to get
back to the sender's local time [1].  With the generator's local time
arbitrarily different from the sender's and viewer's local time,
sticking with UTC seems the best bet.

[1]: https://developer.gnome.org/gmime/stable/gmime-gmime-utils.html#g-mime-utils-header-decode-date