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
```

.gitignore

@@ -17,3 +17,5 @@ tags.*
 !tags.rs
 search_index
 main.css
+*.wasm
+*.js