[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/utils/constants.jsx

@@ -270,7 +270,8 @@ export const PostTypes = {
     DISPLAYNAME_CHANGE: 'system_displayname_change',
     PURPOSE_CHANGE: 'system_purpose_change',
     CHANNEL_DELETED: 'system_channel_deleted',
-    EPHEMERAL: 'system_ephemeral'
+    EPHEMERAL: 'system_ephemeral',
+    REMOVE_LINK_PREVIEW: 'remove_link_preview'
 };
 
 export const StatTypes = keyMirror({