fix: Double signature in composed Email

Re-do of https://github.com/frappe/frappe/pull/12520
Undone by https://github.com/frappe/frappe/pull/12878

Changes done to reflect current state of version-13

Co-authored-by: Suraj Shetty <surajshetty3416@gmail.com>

frappe/public/js/frappe/views/communication.js

@@ -750,7 +750,7 @@ frappe.views.CommunicationComposer = class {
 			signature = signature.replace(/\n/g, "<br>");
 		}
 
-		return "<br>" + signature;
+		return "<br><!-- signature-included -->" + signature;
 	}
 
 	get_earlier_reply() {