Add support for more tile styles

[iomn]

First thought was to use one of basemap-styles. Unfortunately they don't work with our infrastructure (not using tilemill but osm2pgsql+mapnik). General info about carto css can be found here:

• https://ircama.github.io/osm-carto-tutorials/editing-guidelines/
• https://ircama.github.io/osm-carto-tutorials/osm-rendering-process/

osm2pgsql ready styles are the following:

• https://github.com/gravitystorm/openstreetmap-carto (osm default - currently used)
• https://github.com/mapbox/osm-bright
• https://github.com/stekhn/blossom
• https://github.com/kartotherian/meddo (wikipedia maps)
• https://github.com/cybercomsweden/osm-night
• https://github.com/hotosm/HDM-CartoCSS (humanitarian)
• https://github.com/makinacorpus/openstreetmap-carto-bw
• https://github.com/giggls/openstreetmap-carto-de
• https://github.com/MurzNN/openstreetmap-carto-gray-lite

Maybe we could use osm-bright, blossom and gray-lite:

• osm-bright
• blossom

• gray-lite

  A dark version would still be cool.