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/main/layouts.js

@@ -1,5 +1,13 @@
-Meteor.subscribe('boards');
+Meteor.subscribe('boards')
+
+BlazeLayout.setRoot('body')
 
 Template.userFormsLayout.onRendered(function() {
-  EscapeActions.executeAll();
-});
+  EscapeActions.executeAll()
+})
+
+Template.defaultLayout.events({
+  'click .js-close-modal': () => {
+    Modal.close()
+  }
+})