add funtionality to import zip/tgz with maildirs/mboxes to account page

so users can easily take their email out of somewhere else, and import it into mox.

this goes a little way to give feedback as the import progresses: upload
progress is shown (surprisingly, browsers aren't doing this...), imported
mailboxes/messages are counted (batched) and import issues/warnings are
displayed, all sent over an SSE connection. an import token is stored in
sessionstorage. if you reload the page (e.g. after a connection error), the
browser will reconnect to the running import and show its progress again. and
you can just abort the import before it is finished and committed, and nothing
will have changed.

this also imports flags/keywords from mbox files.

serve.go

@@ -134,10 +134,6 @@ requested, other TLS certificates are requested on demand.
 	}
 	mox.MustLoadConfig()
 
-	mox.Shutdown = make(chan struct{})
-	servectx, servecancel := context.WithCancel(context.Background())
-	mox.Context = servectx
-
 	mlog.Logfmt = true
 	log := mlog.New("serve")
 
@@ -163,28 +159,33 @@ requested, other TLS certificates are requested on demand.
 	log.Print("starting up", mlog.Field("version", moxvar.Version))
 
 	shutdown := func() {
-		// We indicate we are shutting down. Causes new connections and new SMTP commands to be rejected. Should stop active connections pretty quickly.
-		close(mox.Shutdown)
+		// We indicate we are shutting down. Causes new connections and new SMTP commands
+		// to be rejected. Should stop active connections pretty quickly.
+		mox.ShutdownCancel()
 
 		// Now we are going to wait for all connections to be gone, up to a timeout.
 		done := mox.Connections.Done()
+		second := time.Tick(time.Second)
 		select {
 		case <-done:
-			log.Print("clean shutdown")
+			log.Print("connections shutdown, waiting until 1 second passed")
+			<-second
 
 		case <-time.Tick(3 * time.Second):
-			// We now cancel all pending operations, and set an immediate deadline on sockets. Should get us a clean shutdown relatively quickly.
-			servecancel()
+			// We now cancel all pending operations, and set an immediate deadline on sockets.
+			// Should get us a clean shutdown relatively quickly.
+			mox.ContextCancel()
 			mox.Connections.Shutdown()
 
+			second := time.Tick(time.Second)
 			select {
 			case <-done:
-				log.Print("no more connections, shutdown is clean")
-			case <-time.Tick(time.Second):
+				log.Print("no more connections, shutdown is clean, waiting until 1 second passed")
+				<-second // Still wait for second, giving processes like imports a chance to clean up.
+			case <-second:
 				log.Print("shutting down with pending sockets")
 			}
 		}
-		servecancel() // Keep go vet happy.
 		if err := os.Remove(mox.DataDirPath("ctl")); err != nil {
 			log.Errorx("removing ctl unix domain socket during shutdown", err)
 		}