test/common: copy shouldFailMixin from tahoe
relay: require valid+signed messages, ignore duplicates
invitation: log message (in twistd.log) when the process completes
add "petmail print-baseurl", for automation
reduce web-polling interval to once-per-2sec

Until we get Server-Sent-Events, local testing is really annoying
without this. We'll crank it back up to something reasonable later.
Really, if the relay can't do SSE, we'll want to poll maybe */15min
until we see a bite, then switch to */2sec.
Makefile: set up a Relay too