Switch email sending go through a queue table in the database

Import the code from the PostgreSQL Europe website to handle this, since it's
well proven by now.

Any points that send email now just write them to the database using the
functions in queuedmail.util. This means we can now submit notification
emails and such things within transactions and have them properly roll bcak
if something goes wrong (so no more incorrect notifications when there is
a database error).

These emails are picked up by a cronjob that runs frequently (typically
once per minute or once every 2 minutes) that submits them to the local
mailserver. By doing it out of line, this gives us a much better way of
dealing with cases where mail delivery is really slow.

The submission from the cronjob is now done with smtp to localhost instead
of opening a pipe to the sendmail command - though this should have no
major effects on anything.

This also removes the setting SUPPRESS_NOTIFICATIONS, as no notifications
are actually ever sent unless the cronjob is run. On development systems
they will just go into the queuedmail table, and can be deleted from there.