Relax jshint when using [] notation unnecessarily

:koala:

Relax jshint when using [] notation unnecessarily by setting the [sub](http://www.jshint.com/docs/options/#sub) option.

Not sure what's going on with `nil`s, empty arrays and the flattens from the source validation, but that's another issue.

/cc @zendesk/quokka

### Risks
 - We will no longer warn developers they are doing something they don't have to do