Delete files when delete button pressed

# Goal
At the moment the Delete button on the MBTiles page doesn't actually do anything. It probably should. For that matter, it probably makes sense to allow users to either delete only a given MBTile file, or delete an AOI and all associated files.

The payload names no files or tests; start at the MBTiles page and trace the Delete button entry point. Define the deletion behavior for one MBTile file and for an AOI with its associated files, then verify both actions remove the intended files.

Written by the indexing model from the issue text.