also, adjusted for signs_lib's change of `allow_onpole` (vertical
poles/fences, that is; explicitly set them to `true`, as appropriate, as
signs_lib now defaults to disabled)
Some signs cannot be put on a horizontal yet:
* all small/med/large "highway" signs
* any signs using one of the "_top" models (such as the yellow speed advisory
signs and the "exempt" signs)
* any signs using the slim pole mount (pedestrian crossings)
