Rewrite all frontend javascript to vanilla ES6

[themightychris]

Currently, frontend javascript depends on the Ext JS framework and build tools.

All frontend javascript should be rewritten with ES6 and with no framework for basic DOM manipulation and XHR

Question: does some sort of warning need to be shown for users of browsers that do not support ES6? ES6 support is broad enough that it should not be a concern for people visiting an API portal, and the existing practice of progressive enhancement means the site will still be usable, so this probably is unnecessary