Made showHints work for lots of windows.

[cfraizer]

The function showHints relied on a feature of UnderscoreJS that apparently
LoDash doesn't support. (The callback for groupBy assumed that the index of
the current element would be passed as its second argument. Underscore does
this, but LoDash apparently does not.)

I'm not a great JS programmer. I ended up changing the partitioning to use
JavaScript's built-in Array.reduce and Object.values functions instead of
UnderscoreJS's (now LoDash's) _.groupBy and _.toArray.

[purag]

Hi Colin! Thanks for the contribution.

I actually think we can do something simpler than this. Since lodash no longer passes the index as the second parameter, we'll just have to keep track of it ourselves. This is actually fairly easy thanks to closures in JavaScript. Instead of reworking the whole solution, we can continue to use the convenient lodash functions and just keep track of k like this:

let k = 0;
let lists = _.toArray(_.groupBy(windows, () => k++ % HINT_CHARS.length));

If you want to go ahead and update your pull request with this new method I'd gladly merge it!