nmbug-status: Escape &, <, and > in HTML display data

nmbug-status: Escape &, <, and > in HTML display data

'message-id' and 'from' now have sensitive characters escaped using
xml.sax.saxutils.escape [1].  The 'subject' data was already being
converted to a link into Gmane; I've escape()d that too, so it doesn't
need to be handled ain the same block as 'message-id' and 'from'.

This prevents broken HTML by if subjects etc. contain characters that
would otherwise be interpreted as HTML markup.

[1]: http://docs.python.org/3/library/xml.sax.utils.html#xml.sax.saxutils.escape