Support for selecting fields to export for batch CSV generation
users should be able to batch-export a CSV of custom data.
from /search/ endpoint, user selects a number of documents, and clicks "export...". a menu should appear prompting the user to select the fields they wish to include in the CSV. On submission, user POSTs the documents requested and the fields selected to the /cluster endpoint-- this will generate a task; the result of which is the requested CSV, which can be downloaded.
how the list of available fields is exposed via API is TBD...