Replace HTML generation in app.js with templates

[slead]

There are numerous examples where the DOM is built using hard-coded HTML in the JavaScript, eg:

var html = '<li>';
html += '<a href="#" data-toggle="collapse" data-target=".navbase-collapse.in" class="dropdown-item liBasemap" data-type="' + basemap.type + '" data-theme="'
          + basemap.theme + '"' +  'data-basemapId="' + basemap.id + '"';

Replace this with a more robust templating approach such as Handlebars.js