(feature) Import & serve tiles using mapproxy (esp. targetting gpkg & mbtiles files).

[JivanAmara]

During import:

• Grab projection type, resolution via gdal.
• Include that data along with file location in ImportLayer or as part of config passed to handlers.

Add new django-mapproxy handler which:

• Copy the uploaded gpkg to [yet-to-be-chosen-location]
• Create djmp models.Tileset from details collected during import
• Do something to let django-mapproxy know to serve the new layer (Tileset.seed()?)