Skip blank hint paragraphs when clicking Hint about khan-exercises

[jeresig]

It looks like you only ever have two types of hints - regardless of the name. Why not just simplify it and reduce it to two names/divs for substitution?

I'm hesitant to remove empty elements as it's possible that there might be a need for actually having an empty element (let's say a div that's styled to look like a circle, or something).

[osnr]

Hmm. If we fix #37, though, there will be three hints on the inheriting problem types. So some problems (the base types) will have two hints and others will have three hints, and the base types will still encounter this issue.

Could we mark empty elements in problem-specific hints as skippable?

[sophiebits]

You can still have hint1, hint2, hint3, and leave hint3 blank if you want and that would work.

Perhaps the current hint inheritance system doesn't make that much sense and it would be better to have more of a include-type thing where you need to specify all the hints for every problem but you can share specific hints if you want. (Like the current way where it copies, but the other way around.)

[jeresig]

direct-variation has:
relationship
fits

v1-over-v2 has:
transform
fits

inverse-k has:
transform
fits

inverse-variation has:
relationship
fits

v1-v2-over-k has:
transform
fits

k-over-a has:
transform
fits

Currently the hint block looks like this:

<div class="hints">
    <p class="relationship"></p>
    <p class="transform"></p>
    <p class="fits"></p>
</div>

Just change it to:

<div class="hints">
    <p class="relationship-or-transform"></p>
    <p class="fits"></p>
</div>

And update all the relationship or transform hints to use the relationship-or-transform class instead.