Made chapter toggle in book sidebar nav more consistent

- Now has a hover state to match other items.
- Now spans the full sidebar with like other items.
- Also updated chapter-toggle to a chapter-contents component, following
  the newer component system.

resources/js/components/chapter-contents.js

@@ -0,0 +1,37 @@
+import {slideUp, slideDown} from "../services/animations";
+
+/**
+ * @extends {Component}
+ */
+class ChapterContents {
+
+    setup() {
+        this.list = this.$refs.list;
+        this.toggle = this.$refs.toggle;
+
+        this.isOpen = this.toggle.classList.contains('open');
+        this.toggle.addEventListener('click', this.click.bind(this));
+    }
+
+    open() {
+        this.toggle.classList.add('open');
+        this.toggle.setAttribute('aria-expanded', 'true');
+        slideDown(this.list, 180);
+        this.isOpen = true;
+    }
+
+    close() {
+        this.toggle.classList.remove('open');
+        this.toggle.setAttribute('aria-expanded', 'false');
+        slideUp(this.list, 180);
+        this.isOpen = false;
+    }
+
+    click(event) {
+        event.preventDefault();
+        this.isOpen ?  this.close() : this.open();
+    }
+
+}
+
+export default ChapterContents;