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.

.eslintrc

@@ -23,3 +23,5 @@ globals:
 
 rules:
   no-unused-vars: [error, {args: all, argsIgnorePattern: ^_, varsIgnorePattern: ^_, ignoreRestSiblings: true}]
+  prefer-const: [2, {destructuring: all}]
+  no-var: [2]

public/js/draw.js

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