[openstreetmap/openstreetmap-website] Allowed to contribute at Zoom Level 24, but enjoy the results at only 19 (#1672)

[Andy Allan]

> Nothing to do with this repository.

I think we can expand on that slightly, both so that @jidanni has more information and perhaps he can find a way forward. "Teach a man to fish", and all that.

 There are three aspects to increasing the max zoom shown on the main page:

* The style should support a particular zoom 
* The servers should support a particular zoom
* This repo should support a particular zoom

For example, the Thunderforest servers support zoom 22, but in all honesty above around 19 or 20 the cartography doesn't really work currently. This repo uses 18 for Thunderforest layers. It could be increased in [leaflet.osm.js](https://github.com/openstreetmap/openstreetmap-website/blob/33845759acf79dc9a0c8460e6363d767bcd86847/vendor/assets/leaflet/leaflet.osm.js).

With the standard layer, the style works reasonably well up to 19 or 20 (a rough rule of thumb is to check is with the maximum zoom levels handled by the [roads](https://github.com/gravitystorm/openstreetmap-carto/blob/master/roads.mss) layers), but the servers run by the OSMF are [limited to zoom 19](https://github.com/openstreetmap/chef/blob/79d28ce14b4c7cf3d964d07879928cd2811fd01c/roles/tile.rb#L94). It might seem straightforward to bump that up a few levels, but such a change would probably be rejected by the server admins. The currently used technology (mod_tile) stores all rendered tiles on disk before returning them to the client, so someone trying to download e.g. z24 for the UK could cause serious side effects! 

My feeling would be that for OSMF to support z20 or higher would need to wait on a more significant technology shift on the OSMF rendering servers, which would allow rendering ultra high zoom tiles on the fly. This is most likely through server-side rendering from vector tiles. If and when that's available, then this repo can be updated with a new maximum zoom.

I hope you find this a useful explanation.

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