Add configuration option for tile bbox

[matkoniecz]

partially addresses #1284

tested with "bbox": [-13821, 6707971, -7886, 6711363]

[matkoniecz]

I considered also more verbose config format, with values like tile_bbox_epsg_3857_south_limit, tile_bbox_epsg_3857_north_limit, tile_bbox_epsg_3857_east_limit, tile_bbox_epsg_3857_west_limit

[matkoniecz]

@Ruben1729 review is welcome!

[KoaCWells]

@matkoniecz May I ask what the intention of the bounding box is? From my impression so far, each Colouring City project should only have building polygons that belong to their corresponding city so I am having a bit of trouble understanding why it is used. Is there some sort of benefit in terms of performance or necessity? We were able to remove the bounding box entirely from Colouring Montreal without any issues so far. Thanks in advance for your feedback!

[matkoniecz]

@KoaCWells Without this bounding box panning across world will create empty tiles that will be then cached.

Maybe extra complexity added by this option and small space saved is making it not worth it? It may be useful to check how much space is saved in the best case (someone dragging map across entire world, at every zoom level).