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
```

Cargo.toml

@@ -72,4 +72,4 @@ postgres = ["plume-models/postgres", "diesel/postgres"]
 sqlite = ["plume-models/sqlite", "diesel/sqlite"]
 
 [workspace]
-members = ["plume-api", "plume-cli", "plume-models", "plume-common"]
+members = ["plume-api", "plume-cli", "plume-models", "plume-common", "plume-front"]