Adding address sitemaps

**This should not be merged now.** I first want to test the general sitemap service in production before merging this.

Anyhow, this is ready to be reviewed. It basically replaces https://github.com/geoadmin/mf-geoadmin3/pull/1448/files 

1. Sitemaps containing all adresses of our wohnungsregister layers. It contains links as [1]. These xml files have 5000 links each (~1MB raw, ~50KB gzipped)
2. Replaces `&` with `&amp;` to have fully valid xml
2. Removes priority from all sitemaps

[1] http://map.geo.admin.ch/?ch.bfs.gebaeude_wohnungs_register=1761259_0&X=199101.00&Y=626766.00&zoom=9
It seems to me that `self._multipart` will always be `None`...