[PLT-1249] Add close button 'x' to the right of a link preview (#7017)

* add close button 'x' to the right of a link preview

* Updating webhook UI

* UI improvements for close button

* Adding hover state

* Making the close button visible on mobile

* previews are permanently disabled/closed for that link

* make post as required props

* fix JS error of undefined

* fix update issue both at center and RHS view

webapp/sass/responsive/_mobile.scss

@@ -1349,6 +1349,16 @@
 
     .post {
         .attachment {
+            .attachment__body__wrap {
+                .btn-close {
+                    height: 30px;
+                    left: -15px;
+                    top: 7px;
+                    visibility: visible;
+                    width: 30px;
+                }
+            }
+
             .attachment__image {
                 &.attachment__image--openraph {
                     max-width: 200px;