Use Rust for the front-end too (#374)

Rust can compile to WASM, so let's use it for front-end code as well.

To compile the front-end:

```
cargo install cargo-web
cargo web deploy -p plume-front
```

templates/base.rs.html

@@ -71,6 +71,6 @@
                 <a href="@uri!(instance::admin)">@i18n!(ctx.1, "Administration")</a>
             }
         </footer>
-        <script src="@uri!(static_files: file = "js/menu.js")"></script>
+        <script src="@uri!(static_files: file = "plume-front.js")"></script>
     </body>
 </html>