Add support for changing background map service in HTML

[jampukka]

Should fix #133

• Add two new properties usable in featureCollection and feature HTML templates
  • settings.tileUrl
  • settings.tileAttribution
• Change the built-in HTML template files featureCollection.ftl and feature.ftl to use said properties
• The value of these properties can be configured via formats.html.tileUrl and formats.html.tileAttribution (see example below)
  • If nothing is configured the values default to OpenStreetMap tileUrl and tileAttribution
• Does not affect the built-in templates featureCollection_3067.ftl or feature_3067.ftl
  • Current solution of custom template per coordinate reference system isn't scalable, requires separate attention in the future

Example configuration snippet:

formats=json,html
formats.html.tileUrl=https://avoin-karttakuva.maanmittauslaitos.fi/avoin/wmts/1.0.0/taustakartta/default/WGS84_Pseudo-Mercator/{z}/{y}/{x}.png?api-key=<your api key>
formats.html.tileAttribution=&copy; <a href="https://www.nls.fi">National Land Survey of Finland</a>