Add "permalink" attribute to project settings

[dkastl]

There was some discussion about how to deal with default zoom level and position:
https://hub.georepublic.net/gtt/redmine_gtt/issues/53#note_3548

Because just recently a city requested to make the city hall the default map location, it's probably a good idea to make this configurable as project settings.

• Possible names could be geohash or permalink.
• The attribute would hold a string containing information like zoom, lat, lon (eventually others), that are seperated with /. Example: 19/34.74701/135.35740
• The field value could be set through the map
• Add to projects API output

[dkastl]

This can be done through OL-ext: https://viglino.github.io/ol-ext/examples/control/map.control.permalink.html

[dkastl]

There is a logic conflict between the use of Permalink and "zoom to extent", which is used whenever possible and which overrides the Permalink control behaviour.

I think both controls cannot be run at the same time.