New dialogs for asking about Preview mode

src/Umbraco.Core/Constants-Web.cs

@@ -17,6 +17,10 @@
             /// The preview cookie name
             /// </summary>
             public const string PreviewCookieName = "UMB_PREVIEW";
+            /// <summary>
+            /// Client-side cookie that determins wether the user has accepted to be in Preview Mode when visiting the website.
+            /// </summary>
+            public const string AcceptPreviewCookieName = "UMB-WEBSITE-PREVIEW-ACCEPT";
 
             public const string InstallerCookieName = "umb_installId";
         }