Permit selection of an existing (already uploaded) AOI for additional MBTiles

# Goal
Users may want to create multiple MBTile files for a single AOI. Maybe because they want multiple versions, and maybe because they weren't happy with the results and want to try again with different parameters. At the moment the only way to do so is to upload the same AOI multiple times.

It would make sense to make it possible to select an existing AOI from the MBTiles page, and then have the choice to either download it (currently the only action as a result of clicking an AOI) or return back to the main page with the selected AOI already cued up as the input.