How to link to a dataset in a certain mag?

[philippotto]

The URL currently encodes the current zoom value, but that value does not fully control the rendered magnification. If a layer only exists in a certain mag, it would be good to have the possibility to create a link so that layer is definitely visible.

Based on #5730, we could encode a certain mag in the URL.

Alternatively, #5628 would also be a good approach to tackle this. However, the user would still need to control this somehow (i.e., link a certain position, but make the zoom value "automatic").