Add CodeMirror to editor page

[collinmesser]

Type of issue

• Bug
• Feature/Enhancement
• Question

Description

Add CodeMirror to the editor page so esfiddle can do the thing it's made to do... fiddle.
https://codemirror.net/
Here is the previous code mirror implementation if you need inspiration or anything. https://github.com/esfiddle/esfiddle/blob/develop/src/js/index.js Don't feel like we need to match any of this though.

Steps to Reproduce

1. [First Step]
2. [Second Step]
3. [and so on...]

Expected behavior: [What you expected to happen]

Actual behavior: [What actually happened]

Versions

ONLY if different than Master currently.
You can get this information from executing npm version.