Add Discord inbound replies (#15)

Stacks on #14. Mapped Discord admins can reply in a known thread; those messages become site posts and email the homeowner. The bot does not publish a site event, so the reply is not posted back to Discord. Photo attachments are ignored until the upload pipeline lands on this branch.

Reviewed-on: #15
Co-authored-by: codegirl-007 <s.raide@gmail.com>

cmd/server/main.go

@@ -37,7 +37,7 @@ func main() {
 		log.Fatalf("mail: %v", err)
 	}
 	bus := events.New()
-	bot, err := discord.FromEnv(store.NewDiscordLinks(db), bus)
+	bot, err := discord.FromEnv(store.NewDiscordLinks(db), bus, store.NewPostgres(db), notifier)
 	if err != nil {
 		log.Fatalf("discord: %v", err)
 	}