Refactor out use of submission phonenumber field

[spwoodcock]

Follow on from #1383

Describe the bug

• We used the phonenumber field to store the task id.
• Now we have entities, we should be able to get additional fields associated with the entity (with a task_id field, accessible in the odata endpoint).
• We should remove usage of phonenumber, as this was tied to a specific QRCode/Form & now we only need a single QR/Form per project.
• Usage was in the submissions table (SubmissionsTable.tsx) and convert_odk_submission_json_to_geojson in central_crud.

Additional context
Add any other context about the problem here.