Enable amenity=post_box in Things layer

[haansn08]

Post boxes are a popular and important feature to map, so it would be nice to be able to edit them in the new "Things" layer.

According to

StreetComplete/app/src/main/java/de/westnordost/streetcomplete/osm/Things.kt

Line 73 in 9faf0f8

// "post_box", - blocked by https://github.com/streetcomplete/StreetComplete/issues/4916

amenity=post_box has not been enabled, because of issue #4916, but this issue has been closed as completed, so maybe this feature could be enabled now?

[FloEdelmann]

#4916 has been closed as completed because it is an issue in osmfeatures and westnordost/osmfeatures#23 has been opened, but that one is still open and unresolved, so the issue is still blocked.

[FloEdelmann]

I'll close this as an answered question and add a clarifying comment to #4916.

[matkoniecz]

I also made 3633fe5