Feature/multi metadata cukes

@scotdalton Here's a failing cucumber test that demonstrates the bug I found in the current multiple_metadata_streams branch.

The issue is that with the :heavy_plus_sign: icon. If there is no source metadata, it works as expected, but if source is present, instead of adding an input element, it adds another copy of the source data. I'm guessing the problem is here, but not sure what to do about it:
https://github.com/NYULibraries/ichabod/blob/e9c65d3029f7fee3994db93f2a05917c1f4a3a7e/app/assets/javascripts/ichabod.js.coffee#L46

Inline comments highlight what works and what doesn't. If we can get this one figured out, I can clean this up a bit more, but am happy with these specs. It would also be fairly straightforward for me to add the integration tests if @jgpawletko doesn't get to it today.