Fix oppia#20153: Trim whitespace at beginning and end of input text t…

…o prevent duplicate options being saved (oppia#20685)

* Add function to trim whitespace at beginning and end of input text

* fix syntax

* fix syntax

core/templates/components/ck-editor-helpers/ck-editor-4-rte.component.ts

@@ -524,6 +524,7 @@ export class CkEditor4RteComponent
           break;
         }
       }
+      textElt.forEach(node => this.trimTextNodes(node));
       let html = elt.html();
       this.value = html;
       // Refer to the note at the top of the file for the reason behind replace.
@@ -586,6 +587,14 @@ export class CkEditor4RteComponent
     this.ck.destroy();
     this.subscriptions.unsubscribe();
   }
+
+  trimTextNodes(node: Node): void {
+    if (node.nodeType === Node.TEXT_NODE) {
+      node.nodeValue = node.nodeValue.trim();
+    } else {
+      node.childNodes.forEach(child => this.trimTextNodes(child));
+    }
+  }
 }
 
 angular.module('oppia').directive(