Implement common logic for saving to disk & writing to db for arcgis_feature_layer and comapeo_observations

[rudokemper]

Goal

Towards #86.

Screenshots

For CoMapeo, a GeoJSON file is now saved in the same location as attachments

What I changed

• Rename save_export_file to save_data_to_file to clarify that it is a generic function that can be used at any time to save data to disk, not only when exporting.
• The ArcGIS Feature Layer script now uses this save_data_to_file function instead of providing its own bespoke logic to do so.
• The CoMapeo Observations script now implements both save_data_to_file and save_geojson_to_postgres.
  • download_and_transform_comapeo_data now transforms the incoming CoMapeo Archive Server data into a GeoJSON Feature Collection.
  • CoMapeoDBWriter class was removed entirely. Any business logic e.g. about converting docId was moved to download_and_transform_comapeo_data. Also, one line of code about truncating table names was moved to the GeoJSON-to-Potgres script (as it is good practice for any usage of this script).

What I'm not doing here

• Adapting the Locus Map script, which will be a decent chunk of work (due to working with 3 different file formats) so will be a separate PR.