VEHPI-31: GPX Track File Clearance

To avoid the SSD filling up, old GPX track files should be removed once uploaded to the API (and their corresponding entries purged from the SQLite DB).

Basic plan is to follow the following procedure

- When uploading the GPX file, the resulting Job ID returned by the API should be written to the database
- Periodically (probably after each GPX upload run), the listjobs method should be called to see whether the import was successful
- If not, requeue for upload. If so, remove the track file and tidy the DB