Wildwatch Kenya: fix Student Assignments's data downloads

[shaunanoordin]

PR Overview

This fixes some issues with the Student assignment CSV downloads ("My Data") on Wildwatch Kenya.

• Issue 1: previously, downloaded Student assignment CSVs would be missing several fields/columns. This was due to incorrect SQL columns being specified, resulting in a silent error where no Subject data was available.
  • External fix required: Kenya database needs to have list of Subjects updated to match current production data
• Issue 2: previously, attempting to download data for an Assignment with 0 Classification would lead to a silent error. Now, a CSV file is downloaded with the text "no data"
• Tweak 1: when asking the SQL server for Subject data, Wildwatch Kenya now specifically asks for specific Subject IDs instead of asking for e.g. a 50,000 item data dump.
• This PR is related to WildCam Lab - "Download My Data" is missing several fields #231 - this PR only fixes the issues on Wildwatch Kenya, similar fixes may need to be applied to other Labs.
• Page affected: https://classroom.zooniverse.org/#/wildwatch-kenya-lab/students , "My Data" download buttons (appears only if you're logged in and have an Assignment)

This PR will be merged and deployed immediately.