Add button to toggle whitespace changes

Closes #4271 

**Merge Request**

<img src="/uploads/ea857269faed38a25b63b768d7f3a85b/Screen_Shot_2016-03-23_at_11.32.41_AM.png" width="1157" />


**Diff view**

<img src="/uploads/bc2ac3e4855584f9214c8f3ddf325b06/Screen_Shot_2016-03-23_at_11.32.53_AM.png" width="1211" />

See merge request !3367