Add "Hint" feature to the Periodic Table Quiz

[HorusGoul]

Add a new setting in the Periodic Table Quiz to enable hints.

If the setting is enabled and a person cannot locate an element on the first try, a visual indicator will pop-up to show them the element's place in the table for a few seconds.

We need to A/B test whether to enable this setting by default or not. Based on my personal preference, I think it would be a good idea to enable it by default.