Implement a modal system

I decided to create my own and not to use a community package, because
1. it's straightforward
2. it's better integrated with our others libs such as EscapeActions
3. monitoring third-party packages evolutions (eg, CSS changes) is a
   lot of work.

This is basically the same rationale than for our other generic UI
components such as the Popup/Popover.

This commit also slightly modify the general layout to remove
unnecessary wrapper DOM nodes.

client/components/lists/main.js

@@ -30,7 +30,7 @@ BlazeComponent.extendComponent({
     $cards.sortable({
       connectWith: '.js-minicards',
       tolerance: 'pointer',
-      appendTo: '#surface',
+      appendTo: 'body',
       helper: function(evt, item) {
         var helper = item.clone();
         if (MultiSelection.isActive()) {