Content block assumes API URL

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.

Locate the content block code that constructs the REST API request and read how the topic details are fetched. Check the available @wordpress/api-fetch usage, then verify the request works when the site uses a subdirectory or does not use HTTPS; done means the block no longer assumes the hard-coded API path.

Written by the indexing model from the issue text.