Fix some RAWR-related TODOs

[zerebubuth]

• Add support for stations being multipolygon relations (already had support for nodes and ways).
• When assigning names to one of pois, landuse, buildings, assign all name variants and translations, not just name itself.
• Remove the hard-coded OSM "source" from RAWR, and make it something that is provided by the table and configurable.

Note that I initially tried to make "source" an Enum, but this turned out to create more difficulties than it solved, as it made it much harder to inject custom, unique sources for testing purposes. It would also make it very difficult to extend vector-datasource or 3rd party styles to additional data sources. Instead, I've replaced it with a Source namedtuple which contains both the short name (e.g: osm) and the long name that we use as a tag value (e.g: openstreetmap.org).