Frontend implementation for multiple feature selection within single form submission

Is your feature request related to a problem? Please describe.

Follow on from #1653, as the frontend implementation.

Issue was described across these links, but consolidating info here.

Describe the solution you'd like

During project creation we need:

[ ] update frontend to allow uploading supplementary geojson geometries
- There should be an additional button 'Upload Additional Features'.
- It should use a checkbox, with the upload field hidden until the box is checked. Similar to the custom XLSForm upload.
- It would probably be good if we can validate the GeoJSON file, in similar way that we validate the AOI area upload (probably not requiring backend).
- The GeoJSON data and filename (without extension) should be saved in frontend state.
[ ] On the final project submit we need to call the /projects/{project_id}/additional-entity endpoint, passing in the GeoJSON file as form data. It should be called after task areas have been created (via API call) and data extract uploaded (via API call), but before the /generate-project-data endpoint is called (this will create the Entity list).
- The param for the API is geojson.
[ ] Pass the filename (without file extension) as the additional_entities param on /generate-project-data.
- This is a list, so would be additional_entities: [GEOJSON_FILENAME_WITHOUT_EXTENSION].
- The endpoint should receive the additional list and inject variables into the XLSForm so the user has a question to select the geometry.

hotosm / fmtm