Playground - Making sure blockMap should always visible by default
Playground - Making sure blockMap should always visible by default

Summary:
Will be iterating a bit on the playground submitting a series of small PR's

- Making sure that blockMap node is always open on the preview pane since
it's most likely one of the first debugging things we will be looking out for
- running prettier on the app.js file
Closes https://github.com/facebook/draft-js/pull/1665

Differential Revision: D7094842

fbshipit-source-id: 8666a185399135a38d4bed315bfff290f718cdf7

facebook/draft-js