Replace react-codemirror with react-codemirror2

[n-riesco]

• react-codemirror triggers a version warning with react@16
• react-codemirror doesn't seem to be maintained

@chriddyp @shannonlal Shall we use https://www.npmjs.com/package/react-codemirror2 ?

[n-riesco]

See #311 (comment) to implement resize.