Get spots left from stripe

[raae]

Inform users of how many spots are left and also disable the buttons when there are no spots left.

• Create netlify function endpoint to get sku's sold count @raae
• Fetch cold count from netlify function endpoint and update state @raae
• Use updated soldCountBySku state to calculate spots left @olavea

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• Continue on the branch I started

git fetch origin
git checkout -b sku-price-spots-#47 origin/sku-price-spots-#47
or
git branch --set-upstream-to=origin/sku-price-spots-#47

and
git pull

• See comments inside AngelsCheckout.js

With yarn develop, the soldCount will never be updated, it will be set to 0 (see line 60). This ok for developing, so make that work first.

Later you will use netlify dev to also test against the netlify function returning actual sold counts.