Add post offices

[wmisener]

Makes progress on #435 and #692. Renders post offices (amenity=post_office, aka in OMT class=post + subclass=post_office) with an envelope icon. This is quite similar to the Carto and Maki approaches.

I've sometimes seen American maps render an envelope with the bottom folds as well, making more of an 'x' shape. See e.g. the NPS or the MUTCD. I'm open to this style of envelope, but in the end I didn't think that the extra visual clutter made it that much more identifiable.

The icon comes in at z15, the same as other 'community anchor' POIs. I went with blue color, since I see post offices as more of a community service provider than a consumer destination.

Unfortunately, in my opinion, there is no information in the tiles we can reliably use to distinguish public post offices (like the USPS) from private ones (UPS Stores, etc), as mentioned in #435 (comment). So this PR renders them both identically, although I see distinguishing them in the future as desirable. This would be a good use case for using operator:type or operator:wikidata were those tags to be exposed in OMT, similar to what's been discussed more broadly for supporting brand logos (#809).

[ZeLonewolf]

Looks great, thanks! Sorry for the delay in reviewing.