Drop all_networks and all_shield_texts from low and mid zooms

[nvkelso]

To decrease file size let's drop all_networks and all_shield_texts from low and mid-zooms when there's often not room to display this information.

• highway: less than zoom 10
• major_road: less than zoom 12
• other kinds: less than zoom 14

NOTE: MVT format currently doesn't include the array objects while GeoJSON and TopoJSON do.

[nvkelso]

No obvious regressions (in MVT) here, closing.

(Would later yield savings if we write out nested props as strings into MVT like some software does.)