Fix #217

When the same icon is requested several times (like a logo in the header and the footer) there is a race condition in the decorateIcons function that will cause only the first request to get the actual svg while the other requests will get empty icon tags.

This PR fixes this by replacing the current logic that is based on just putting a boolean in the cache while the icon gets loaded with a promise that is put into the cache (similar to what happens in fetchPlaceholders). Subsequently, that promise is returned from the cache for all requesters - hence, they all get the result.

We used this patch in our project and haven't had any issues since then.

Test URLs:

• Before: https://main--helix-project-boilerplate--adobe.hlx.live/
• After: https://217--helix-project-boilerplate--karlpauls.hlx.live/