Add linter rule disallowing use of var

var in javascript has some surprising behaviors, especially for those
of us from a python background, and can hide some errors. There's a
good article on it here: https://tylermcginnis.com/var-let-const/

Tell eslint to not allow the use of var and instead require either
let or const. Then update the one use.

Change-Id: Idd4b0a9086a68070dc35b546b7f73701c94ba1b0

web/.eslintrc

@@ -3,6 +3,7 @@ plugins:
   - standard
   - jest
 rules:
+  no-var: error
   no-console: off
   semi: [error, never]
   quotes: [error, single]