feat(mail(web)): Templates folder for pre-defined drafts

To create a template, the user needs to:

 - define a mailbox as the Templates folder;
 - move a draft to this special folder.

Fixes #4320
Fixes #5363

UI/WebServerResources/js/Mailer/MessageController.js

@@ -479,6 +479,12 @@
       }
     };
 
+    this.compose = function($event) {
+      if (!this._showMailEditorInPopup('compose')) {
+        _showMailEditor($event, this.message.$compose());
+      }
+    };
+
     this.openInPopup = function(action) {
       var url = [sgSettings.baseURL(),
                  'UIxMailPopupView#!/Mail',