convert all js var to let/const (#7464)

Most of the work was done by `eslint --fix`, I fixed around 10 cases
where it could not auto-convert.

public/js/draw.js

@@ -1,7 +1,7 @@
 /* globals gitGraph */
 
 $(document).ready(function () {
-	var graphList = [];
+	const graphList = [];
 
 	if (!document.getElementById('graph-canvas')) {
 		return;