Content block assumes API URL

[johngodley]

The content block makes request to the REST API to get topic details. However, this assumes the path:

https://${ blog }/wp-json/wp/v2/topic?slug=${ encodeURIComponent( slug ) }

It's possible the path is different. For example, if the site is installed in a subdirectory or doesn't have https enabled.

If we use @wordpress/api-fetch then it already knows the REST API URL.