Compare at price logic

[NathanPJF]

What are you trying to accomplish with this PR?

Fixes #96

• Always render markup for compare at price, so JS can update it.
• On featured product page, use current_variant information so the initial prices displayed reflect the option selectors.

New stuff:
The changes in product.js have to do with how we are handling compare_at information when the variant is on sale. We are always leaving the "compare_at" text on screen for screenreaders, even when we were hiding the sale price. Now it hides that text if the variant isn't on sale.

I also update the strikethrough price when the variant isn't on sale. It was already being hidden with the hide class, but it just felt odd to leave a strikethrough price in the markup if we had no intention of using it.

/cc @Shopify/themes-fed

Checklist

For contributors:

• I have updated the docs to reflect these changes, if applicable.

For maintainers:

• I have 🎩'd these changes.
• I have bumped the package.json version in a separate PR, if applicable.

[NathanPJF]

Same approach approved here: https://github.com/Shopify/narrative/pull/114

[lock]

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