API Editor: Replace Ace editor with Codemirror

[EricWittmann]

Hopefully this will eliminate the problem with the Ace editor causing conflicts with the drop-down component.

[EricWittmann]

I have an initial implementation of this, but the CodeMirror editor simply doesn't integrate as well as Ace. In addition, the look & feel of Ace is better, and it has better features. But here is the potential implementation:

https://github.com/Apicurio/apicurio-studio/tree/proposal/code-mirror

The core problem with Ace remains: as soon as an Ace editor is added to the page, all drop-down controls become very difficult to click on (the drop-down items become hard to select).

[EricWittmann]

This is no longer necessary as I have fixed the problems with the ace editor.