Review access permissions on footways

[ansoncfit]

We apply foot=yes access permissions to all ways tagged footway at https://github.com/conveyal/r5/blob/v6.8/src/main/java/com/conveyal/r5/labeling/TraversalPermissionLabeler.java#L63

But OSM does have footways with access=no (implicitly for all modes), so we might not want to always override that with our own mode-specific foot=yes. For example, see this emergency walkway along high speed rail tracks). As of #841 / #848, this walkway is disconnected from the rest of the network thanks to properly tagged emergency exit nodes.