Fix more prefer-const warnings

[GoodForOneFare]

The js-codemod/no-vars transform should be picking these off, but there are many left behind in places that should use const.

For example, in product-resource-browser.js, this loop should use a const for the variant variable in its for loop:

for (let variant of product.variants) {
  variant.disabled = this._variantDisabled(variant, product);
  variant.alreadyAdded = this._variantSelected(variant);

  if ((product.alreadyAdded === null || product.alreadyAdded) && !variant.disabled) {
    product.alreadyAdded = variant.alreadyAdded;
  }
}

[lemonmade]

I have noticed this mostly in for of loops, I think there must be some logic off in the transform we use for this specific case.

[lemonmade]

Fix submitted to js-codemod: cpojer/js-codemod#43