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,

ui/elm-package.json

@@ -14,6 +14,9 @@
       },
       "/debug": {
         "target": "http://localhost:9000"
+      },
+      "/serve": {
+        "target": "http://localhost:9000"
       }
     },
     "dependencies": {