MM-11434 Only call PreparePostForClient once when creating a post (#9868) (#9876)

* MM-11434 Only call PreparePostForClient once when creating a post

* Have PreparePostForClient provide new metadata when a post already has it and update tests

api4/post.go

@@ -67,7 +67,9 @@ func createPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.App.UpdateLastActivityAtIfNeeded(c.Session)
 
 	w.WriteHeader(http.StatusCreated)
-	w.Write([]byte(c.App.PreparePostForClient(rp).ToJson()))
+
+	// Note that rp has already had PreparePostForClient called on it by App.CreatePost
+	w.Write([]byte(rp.ToJson()))
 }
 
 func createEphemeralPost(c *Context, w http.ResponseWriter, r *http.Request) {