Make the key field input into a dropdown

[waxlamp]

This is the UI element introduced in #54. It should be a dropdown since the choice of key fields is constrained to the existing headers in the file to be uploaded.

[waxlamp]

@AlmightyYakob wrote:

The question is should we do the parsing of that server-side or client-side.

Good question. My intuition is that we should do minimal processing clientside to extract the column titles, using something proven like Papa Parse. We could roundtrip to the server for this, but that seems like overkill.