Clean up JS code / add JS linters to Travis

[orenyk]

Offshoot of #1075, we need to clean up our JS code such that it passes our JS linters before we include those linters in Travis. This should probably happen after #1061 #245 since there will likely be significant changes to our JS codebase then.

[mnquintana]

FWIW I don't think we need to wait on #1061 – that will be so much of a rewrite that it's a long ways off. We would really benefit from cleaned up JS now, even if much of it will be rewritten in several months.

I do think we should wait on #245 though, since that will be done a lot sooner.

[orenyk]

Good points, I'll update the description :-).

[orenyk]

Merged into #1581