Change var to let, and use object destructuring, in a couple of previously class converted web/*.js files

Note that these files were among the first to be converted to ES6 classes, so it probably makes sense to do another pass to bring them inline with the most recent ES6 conversions.

web/password_prompt.js

@@ -87,7 +87,7 @@ class PasswordPrompt {
   }
 
   verify() {
-    var password = this.input.value;
+    let password = this.input.value;
     if (password && password.length > 0) {
       this.close();
       return this.updateCallback(password);