Hosted overlays take PMTiles up to 50 MB

Bring-your-own-data overlays now accept PMTiles files up to ten times the old limit, while GeoJSON overlays stay at 5 MB.

• Hosted overlays (/byod, and the dashboard's data page) now take PMTiles files up to 50 MB. GeoJSON overlays stay at 5 MB.
• GeoJSON stays small because a map downloads it whole. A PMTiles archive is read a tile at a time, so a large one costs a map no more than a small one.
• The ten-overlay-per-account cap is unchanged.