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
```

static/js/autoExpand.js

@@ -1,7 +0,0 @@
-const articleContent = document.querySelector('#plume-editor')
-const offset = articleContent.offsetHeight - articleContent.clientHeight
-
-articleContent.addEventListener('keydown', () => {
-  articleContent.style.height = 'auto'
-  articleContent.style.height = `${articleContent.scrollHeight - offset}px`
-})

static/js/menu.js

@@ -1,13 +0,0 @@
-// It should normally be working fine even without this code
-// But :focus-within is not yet supported by Webkit/Blink
-
-const button = document.getElementById('menu')
-const menu = document.getElementById('content')
-
-button.addEventListener('click', () => {
-  menu.classList.add('show')
-})
-
-menu.addEventListener('click', () => {
-  menu.classList.remove('show')
-})

static/js/search.js

@@ -1,13 +0,0 @@
-window.onload = function(evt) {
-  var form = document.getElementById('form');
-  form.addEventListener('submit', function () {
-    for (var input of form.getElementsByTagName('input')) {
-      if (input.name === '') {
-        input.name = input.id
-      }
-      if (input.name && !input.value) {
-        input.name = '';
-      }
-    }
-  });
-}