ui: Much elm work, such wow

- ui: Fix favicon
- webui: Changes to support serving Elm UI
- Static files now served from `/` mount point.
- Old UI handlers moved to `/serve` mount point, some will still be
  needed by the Elm UI; safe HTML and attachments for example.
- Update dev-start.sh for new UI, with tip on how to build it.
- ui: Detect browser host:port for websocket URL,
- webui: Remove unused mailbox handlers, rename routes
- Many routes not needed by Elm UI.
- `/serve/mailbox/*` becomes `/serve/m/*`.
- webui: Impl custom JSON message API for web UI,
- ui: Refactor Mailbox view functions,
- ui: Add body tabs for safe HTML and plain text,
- webui: Format plain text for new UI,
- ui: List attachments with view & download links,

cmd/inbucket/main.go

@@ -124,9 +124,9 @@ func main() {
 	retentionScanner := storage.NewRetentionScanner(conf.Storage, store, shutdownChan)
 	retentionScanner.Start()
 	// Start HTTP server.
-	web.Initialize(conf, shutdownChan, mmanager, msgHub)
+	webui.SetupRoutes(web.Router.PathPrefix("/serve/").Subrouter())
 	rest.SetupRoutes(web.Router.PathPrefix("/api/").Subrouter())
-	webui.SetupRoutes(web.Router)
+	web.Initialize(conf, shutdownChan, mmanager, msgHub)
 	go web.Start(rootCtx)
 	// Start POP3 server.
 	pop3Server := pop3.New(conf.POP3, shutdownChan, store)