Jobs fixes

This adds a new Interactive Developer job and removes the JS Engineer role, but also fixes a few bugs, namely the devblog vanishing due to CAPI v2 switch, and system JS being updated (at least, when I ran it locally) and all the file references failing.
I assume there's a rate limit on this key? In that case, erroring requests become even more likely and it'd be nice to [handle the failure case](https://github.com/guardian/developers-site/blob/ma-jobs-fixes/src/js/app.js#L45). 

Doubt you'd be able to fetch the RSS as it'd be cross-origin and probably doesn't have the headers setup.