Download Data page needs a little UX love

[Viqsi]

So there's a few issues with Download Data:

• "Two trash cans". See #234. There's one for "remove from download" and one for "delete item" - and this happens for both variable sets AND cohorts. Not Great.
• We really need some way of notifying the progress spinner that download has started. That might require revamping how a download file is sent to the user tho...
• If there's no results, the user gets a very generic looking page just showing {"message":"no results"}, which is Decidedly Unfriendly. Again, this might also require revaming how a download file is sent to the user.

[Viqsi]

The second item might be doable via an event listener; I'd have to look around and see if there's an event fired for "download has begun and user has been prompted" or whatnot.

[Viqsi]

There is no such event and the workarounds are made of failure and sadness, so I've been working on splitting the API endpoint into two - a "create this download" and "retrieve this download" matched set, much like what we do with, well, everything else. Thankfully @WValenti's existing backend works on this model, so it's just a question of modifying the interface to it.