Estimate download size before launching job

There should be an error and/or warning when someone attempts to create too large an MBTile file.

- Either fetch the area of the AOI using GDAL and calculate the number of tiles given the requested zoom levels, or
- Just run the create_csv script and count the tiles. This will still be problematic if someone attempts to generate a CSV with a high zoom level for a whole country—the script will stall or maybe even run out of memory—but in most circumstances it'll be simpler and more accurate (for one thing, it'll give a precise estimate of the number of tiles that will be generated).

Either way, a request that will generate a ludicrously large MBTile file should generate a warning to the user before proceeding.

Start by tracing the job-creation flow and the mentioned create_csv script to determine where requested zoom levels and the AOI are available. Compare estimating tiles through GDAL with counting create_csv output, then verify that an oversized MBTile request is warned about before proceeding.

Written by the indexing model from the issue text.