default to stable sort by properties.datetime, id, collection

[philvarner]

Currently, there is no sort by default. This should be done by properties.datetime, id, collection by default, so that pagination via search_after can be implemented.

sort =[
  { 'properties__datetime': { "order": "desc" } },
  { "id": { "order": 'desc' } },
  { "collection": { "order": 'desc' } }
])