Change multiselect cache structure

[meghaarora42]

The existing structure used an Object as a cache where the keys are the
values of option elements, and values are the HTML content of the option
elements. In Javascript, if a numeric value in the form of a string is
assigned as a key, it gets converted to an integer. Optimization
routines would then order the object to ensure faster access to
elements. For example:

cache = {}
cache['2'] = "two"
cache['1'] = "one"
console.log(cache[2])  #=> "two"
console.log(cache[1])  #=> "one"
console.log(cache)     #=> {1: "one", 2: "two"}

Note the coercion of strings to ints above. This messes with the
ordering of multiselect options list whenever there is a user input. To
avoid this from happening, the keys need to have a string that can't be
coerced automatically, and then preserve the value of the option
element.

I chose to use an object that stores the option value and the option
HTML as the value of the cache and a string of the format 'o_' as the key. This ensures that the insertion order is preserved.
This is the new structure:

cache = {
  'o_271': { id: 271, value: 'CartItem #271'},
  'o_270': { id: 270, value: 'CartItem #270}'
}

[meghaarora42]

Fixes #2231

[mshibuya]

Nice try, thanks RailsGirls 😻

[jordanmaguire]

Thanks for this!

[sferik]

Great work! 🌈 💖 ❤️ 💛 💚 💙 💜 💝

[jordanmaguire]

Here's some thoughts:

1. What's your thoughts on test coverage? This would be super easy to test with capybara and would prevent regressions
2. This code would REALLY benefit from some comments explaining why it is necessary to do what has been done. In 6 months or whenever when someone else is looking at this code there is going to be no indication of what this code does. It's good that the info is in the PR, but some of it should be in the code, too.