Merge #tracks-notunnel-nobridge into #roads-fill

The layer #tracks-notunnel-nobridge has been merged into #roads-fill.

This unifies the various track-definitions.

Rendering changes:
The rendering order of tracks versus other roads is now determined by z_order.
Therefore, tracks are now leveled at the same level as service roads, footways, cycleways, and paths, instead of below them.
The rendering of the tracks themselves has not been altered.
Only render tracks from level 13

Oops, roads should only render from level 13 on.
Fix rendering tracks without tracktype

Make sure that tracks without tracktype render as before merging #tracks-notunnel-nobridge into #roads-fill, by providing defaults.
Merge branch 'roadsv3.0' of https://github.com/math1985/openstreetmap-carto into math1985-roads
Add a guideline on the mapper feedback loop and misspellings.