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/posts/new.rs.html

@@ -70,5 +70,4 @@
             }
         }
     </form>
-    <script src="@uri!(static_files: file = "js/autoExpand.js")"></script>
 })