HOLI-11151: add geolocation_geojson columns to models with geolocation column

Google Datastream is replicating Okunas tables into BigQuery for subsequent processing in data
pipelines. Unfortunately, Datastream does not support Postgis binary column formats. In order to
have geolocation data available anyway, this MR:

* introduces a colocated field `geolocation_geojson` next to every `geolocation` field in every
  model using geolocation as point.
* adds a migration to ensure all `geolocation_geojson` columns are set if `geolocation` is set.
* hides `geolocation_geojson` from admin interfaces