Fix mismatch between GaiaGPS iOS and gaiagps.com as it comes to custom tiles

I was looking at the Big Sur Trail Map, and noticed that the import procedures between iOS and gaiagps.com differ: the former supports the convenient TileJSON format, the latter errors if you try to import it like a GPX file, which is the functional but somewhat obscure UI workaround that GaiaGPS iOS uses. Instead, I must use the old-school macro-substitution URL variant.

Furthermore, upon importing a layer, it is uncharacteristically not synchronized between iOS and gaiagps.com, unlike virtually everything else in Gaia.