Allow users to submit feedback by clicking on labels

[anyweez]

Current design is to create a new /api/feedback endpoint that accepts post requests. Payload would include:

correct: <bool>
docid: <string>
label: <string>     // example: countries
value: <string>    // example: costa rica

Pings are sent from the frontend and keyed off of docid, label, and value. Should be able to tabulate

• coverage (already done), country_label_available / article_count
• precision, of rated labels, what % are correct?
• recall? not yet, need to be able to rate unassigned labels

To compare two implementations, mirror tables in rethinkdb and label using separate processes. compute state for each. TODO: make scripts to help with this last part.

[anyweez]

Success! Also serving them back to the frontend.