Dev Server Should Hit Local Blocks Endpoint

Currently if you open a liveblog on the DCR dev server, the `fetch` requests it makes for new blocks go to PROD rather than the dev server itself. This may make it difficult to develop and test changes to the blocks loading mechanism.

To solve this the local blocks endpoint may need some way to retrieve blocks data from frontend, similar to what the dev server does for full article requests:

https://github.com/guardian/dotcom-rendering/blob/e0aa663ace65c7f50f13d7433a38cdb44acdab6c/dotcom-rendering/webpack/webpack.config.dev-server.js#L60-L62