[MM-42192] Include deleted posts in GetPost (#20358)

* Introduced include_deleted query param on get posts endpoint

* Update the correct func name in the comment.

Co-authored-by: santoniriccardo <santoni.riccardo@gmail.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel.go

@@ -1613,7 +1613,7 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	if ok && len(postRootId) == 26 {
-		rootPost, err := c.App.GetSinglePost(postRootId)
+		rootPost, err := c.App.GetSinglePost(postRootId, false)
 		if err != nil {
 			c.Err = err
 			return