Add platform from modal

[Aaron-Robertson]

Fixes #708

[Aaron-Robertson]

@WrathfulSpatula @vprusso Is the edit button what we have in mind here? I haven't gotten a chance to test yet, but I wonder if a typeahead add new option might be nicer, since we're already in a modal.

[WrathfulSpatula]

@Aaron-Robertson I suspect that we need to open a secondary SubmissionRefsAddModal. Then, when the user creates the platform in that modal, it'll be necessary to automatically add the reference to the current submission, or it won't be possible to key a result to it. (I actually think Will's suggestion in #708 breaks separation of concerns pretty badly, but we decided to bounty the issue, so that's what has to be done: the user opens a new modal, defines the platform, and the front end automatically adds it as a reference to the current submission. Actually, SubmissionRefsAddModal might already add the reference to the current submission.)

[Aaron-Robertson]

Gotcha, modal in modal it is! Do we want to include a dropdown option that says "Add Platform"? Or use the existing EditButton on the "Platform" label?

[WrathfulSpatula]

Users can either add an existing platform or create a new one, from here. So long as both options are available, I trust you to decide on "look-and-feel," whether using EditButton.