slow discussions end point

[camallen]

Reports of slow talk discussion page loads for projects with lots of discussions on a board, e.g. snapshot projects etc. https://www.zooniverse.org/talk/17/1130008?comment=1852795

I believe this may be due to the partial index being missing on the default sort order for the index route:

• https://github.com/zooniverse/Talk-Api/blob/479163176ba01aebd665099de50f4f58f8944be0/db/schema.rb#L123
• https://github.com/zooniverse/Talk-Api/blob/479163176ba01aebd665099de50f4f58f8944be0/app/serializers/discussion_serializer.rb#L11

Need to test out the SQL generated from the serializer and see the explains on this on the production db to determine why these are slow for larger discussions vs smaller ones.