Add the blob async request to StartupJS

Proposal

When rendering the blob content page, we request the pre-rendered markup of the content via an ajax request.

The moment this request is triggered, is way down on the list of requests.

Adding this to the startupjs could add some easy and quick wins in terms of performance improvements in small or large file pages.

Waterfall

Context

This will benefit the current implementation (HAML) before we're able to enable the Vue implementation. Regardless, we should keep this optimisation in the future blob renderer too.