On which map do you want to add a scale control? If you want to add one to the map on an item show page, open mapping-show.js and add your code to the file:
// ...
var map = L.map('mapping-map', {
fullscreenControl: true,
worldCopyJump:true
});
L.control.scale().addTo(map); // add this line
var markers = new L.FeatureGroup();
// ...