Use standard padding size for map-ui sidebar

The previous padding didn't match the main sidebar, and also used
sass division as part of the calculation.

app/assets/javascripts/leaflet.sidebar-pane.js

@@ -20,7 +20,7 @@ L.OSM.sidebarPane = function (options, uiClass, buttonTitle, paneTitle) {
     var $ui = $("<div>")
       .attr("class", uiClass + "-ui");
 
-    $("<div class='sidebar_heading d-flex'>")
+    $("<div class='d-flex p-3 pb-0'>")
       .appendTo($ui)
       .append($("<h4 class='flex-grow-1 text-break'>")
         .text(I18n.t(paneTitle)))