Allow Browse Table Contents to be downloaded

[acoffman]

This is a proof of concept of adding a "download" button to browse tables. There are a couple of tricky parts here:

• We want to respect any filtering the user has done in the downloaded file.
• We can't do this client side super easily as we want all the data, not just the currently displayed, and the client displays things as tags and overflows which aren't really "downloadable" as a TSV
• As we have "infinite scroll" rather than traditional pagination, we want to stream the response so we don't have every single row in memory at once.
• The GQL runtime returns JSON only

What I've done here is add a different non-graphql route to the server. This route will accept requests that look like GraphQL but return TSV instead. This is done utilizing the fact that the SearchObject plugin we use for the browse tables can be used outside the graphql context.

The new controller will grab the variables from the query, pass them into the specified SearchObject and then stream the result back out as a TSV. It basically just requires that you map a table_name to the correct resolver in the hash in the controller, and then you implement table_headers and to_row in the resolver so it knows how to represent a table entry in TSV.

You can see how its pretty straightforward to hook up on the frontend as well.

closes #389 (if/when we implement it on all tables).