Make Ghost client to use Svelte fetch API

[miohtama]

Currently, Ghost client does not use fetch from Svelte. This means that there are no render-side rendering benefits: Ghost API always issues out fetch on the client-side on the page load, because it does not know about the data fetched on the server-side. This should be visible on front page and blog roll by checking the console.

Investigate if this could be easily fixed. If not then let's not build a complex fix, because this is a very low impact issue ATM.