Added a new table fielddefs that records information about the

Added a new table fielddefs that records information about the
different fields we keep an activity log on.  The bugs_activity table
now has a pointer into that table instead of recording the name directly.

Set up a new, highly experimental email-notification scheme.  To turn
it on, the maintainer has to turn on the "New email tech" param, and
then individual users have to turn on the "New email tech" preference.