"Improve this Map" link missing mapid when layers are manually added

[jfirebaugh]

Example: http://lxbarth.com/map/#9/40.0000/-74.5000

The map is initialized without TileJSON and layers are manually added, so when Mapbox.js looks for a mapid it can't find one.

Either the map needs to traverse the layers (and layer groups) looking for specific Mapbox.js classes, and grabbing their mapid (eww) or the _updateMapFeedbackLink code should live in the layers themselves (probably better).

[lxbarth]

Can we prioritize fixing this? @jfirebaugh @mourner @tmcw @tristen - who can take this on?