Add Mormon Pioneer and Pony Express National Historic Trail Auto Tour Route shields

[quincylvania]

These are just plain shield since we don't have permission to use NHT shields yet, but the routes are mapped.

Re: #1101

[quincylvania]

A note of caution, some of these NHT are collinear with long stretches of interstates and US routes, so the shields may show up at relatively low zooms. This is z6 in Utah with this PR. The orange blob looks kind of weird considering there is no graphic.

[1ec5]

I think I’d actually expect that behavior along long concurrencies. Shorter concurrencies would cause more confusion, so OpenMapTiles tries to omit short concurrencies, albeit somewhat clumsily: openmaptiles/openmaptiles#1553. Even if once we get permission to use more accurate NHT shields, users will probably need to consult the legend to decipher the pictogram. Unfortunately, overrideByName isn’t supported yet: #1165. I think a fix for that bug would mitigate user confusion enough to get away with these shield blanks.