Warn before closing if unsaved on edit page

It doesn't fire if you go back to the edit page and the browser
auto-fills the previous contents, but oh well.

static/editor.js

@@ -0,0 +1,14 @@
+(function () {
+	let changed = false;
+	let textarea = document.querySelector('.edit-form__textarea');
+
+	let warnBeforeClosing = function (ev) {
+		ev.preventDefault();
+		return ev.returnValue = 'Are you sure you want to exit? You have unsaved changes.';
+	};
+
+	textarea.addEventListener('input', function () {
+		if (!changed) window.addEventListener('beforeunload', warnBeforeClosing);
+		changed = true;
+	});
+})();