Add a neutral place of worship icon

[wmisener]

In #910, I introduced a "generic" place of worship icon, consisting of just the bottom square that for other religions is topped by a symbol. However, this symbol is never used, meaning that any amenity=place_of_worship without one of the explicitly rendered religion values is not rendered at all.

The reason I did not implement this in #910 is just because I wasn't sure how to do a 'catch-all'/else-type value in poi.js, essentially picking up all possible values of the place_of_worship class without one of the specified eight subclasses. If someone knows how to do that, it would be relatively simple to implement this.

It would also be worth discussing if just a square is sufficient for this purpose. I thought that, while ambiguous on its own, in the context of the map with other places of worship symbols it would be clear enough. But perhaps some neutrally 'religious' symbol would work better.