Improve marsh/wetland/mud rendering

* Render marsh and wetland from z10 (resolves #899)
* Render labels on marsh, wetland and mud (resolves #935)
Change rendering theme park boundaries

Render theme park boundaries in the style of marina boundaries (but brown
instead). This prevents confusion with roads.
add missing space
Split landcover and text-poly in high zoom (>= 10) and low zoom (< z10) layer

This causes significant speed improvements in z7, z8, and z9.
For example, rendering in z7 is now more than 5 times as fast.
Change rendering country/state labels

* Country and state labels are now rendered from the admin borders in the polygons
  table rather than from the place nodes in the points table.
* The maximum zoom level requirement for country/state labels is replaced by a
  maximum area size requirement.
* A minimum area size requirement for country/state labels is added.

Advantages:
* Avoid cluttering of too many labels of small areas, in particular on z4 and z5.
* Easier to detect broken country and state multipolygons.

Disadvantage:
* This requires that country and state multipolygons are not broken, otherwise
  the labels won't render.
Improve SQL layout
fix grave yard naming - fixes #1110
remove obsolete natural=land query - fixes #1106
Change boundary nature reserve / national park areas

This changes the rendering of national parks in the following way:
* Render border with a double uninterrupted line (line marinas).
* Require 100px minimum size for rendering.
* For zoom levels 10-12, drop transparent fill.

* Resolves #69
* Resolves #563
* Supersedes #1077

Changes with respect to #1077:
* Restore transparent fill for zoomlevels 7-9.
* Make line slightly narrower on zoomlevels 10-12 and wider on zoomlevel 10-13.
Minor change in width and opacity themepark boundaries
Slightly darken school text colour

This resolves #1048.
Merge amenity-symbols into amenity-points

This resolves #210
Document #1107

This resolves #1131.
Add building IS NULL clause to low zoom landcover queries

This should improve performance
Add new marsh/wetland icon

Thanks to @imagico for proposing the icon:
https://github.com/gravitystorm/openstreetmap-carto/pull/1042#issuecomment-5
Merge pull request #1152 from math1985/merge-symbols-points

Merge amenity-symbols into amenity-points
Merge pull request #1151 from math1985/document-admin

Document #1107
Merge pull request #1149 from math1985/school-colour

Slightly darken school text colour
Merge pull request #1146 from mkoniecz/land

remove obsolete natural=land query - fixes #1106
Merge pull request #1105 from mkoniecz/ce

refactor and fix amenity-points