Drop polygon-opacity from landcover

Drop polygon-opacity from landcover tags that still had it, i.e.:
* tourism = camp_site
* tourism = caravan_site
* tourism = picnic_site
* landuse = garages
* landuse = fill
* leisure = park
* leisure = recreation_ground
* landuse = brownfield
* landuse = landfill
* landuse = construction
* aeroway = aerodome

Polygon-opacity should be avoided, as it gives smudgy and sometimes unexpected
colours in case of overlapping polygons.

Also colour landuse=field as landuse=farm_land, as the old landuse=field
without opacity is too dark.
Move control over road rendering order to SQL

Currently, rendering order of road rendering within one layer is handled
by the z_order column, which comes from osm2pgsql. As such, we have
little control over road rendering without reloading the database.
This PR moves control over the rendering order to the SQL query.

This adds complexity to the SQL queries, but increases customizability,
and simplifies the roads.mms code.

This solves the following issues:

* #462 (Move rendering order road types from osm2pgsql to our SQL queries)
* #163 (Railways are now drawn above roads)
* #167 (Tramway layering issues)
* #168 (Paths are now drawn below link roads)
* Trac 2024 (Service roads are now rendered below link roads)
* Trac 3649 (Service roads are now rendered below race tracks)
* Pedestrian and living streets are now consistently ordered
* Footways are now always displayed under service ways
Correct rendering lowzoom and names of links

This corrects a mismatch between the queries and the style sheet for
the roads-low-zoom and roads-text-name layers.
This should not have influence on performance.
Remove no longer needed link references
Make colors former transparent landuses darker

The colors have been chosen in such a way that they stay the same on the
gray 'land' background.
Remove irrelevant subway bridge casing change from PR
Make bridge values consistent

Be consistent in the bridge values for which we test.
Note that bridge=true and bridge=1 are no longer in use.
Clean up highway labels

* Make font sizes more consistent (resolves #544)
* Let tertiary labels start at z14 rather than z15 (resolves #301)
* Add label for bridleways
* Remove halo from residential/unclassified/road (resolves #515)
* General code clean-up
Render names of large names at lower zoomlevels

This resolves #700.
Give service roads the same color as residential roads

This resolves #817.
Revert accidental change in dasharray of steps

This solves #760.
Merge branch 'math1985-step-style'
Merge branch 'service-casing-color' of https://github.com/math1985/openstreetmap-carto into math1985-service-casing-color
Merge branch 'math1985-service-casing-color'
Merge pull request #812 from math1985/road-labels

Clean up highway labels
Merge branch 'bridge-references' of https://github.com/math1985/openstreetmap-carto into math1985-bridge-references
Merge branch 'math1985-bridge-references'
Merge branch 'opacity' of https://github.com/math1985/openstreetmap-carto into math1985-opacity
Merge branch 'math1985-opacity'
Merge branch 'lake-names' of https://github.com/math1985/openstreetmap-carto into math1985-lake-names