#10883 adding const error string DismissPostError to be returned by MessageWillBePosted (#10921)

Adds an exported const string that can be used by plugins to signal a client that the incoming post should be dismissed, instead of the current behavior which just leaves it pending.

store/sqlstore/post_store.go

@@ -296,6 +296,7 @@ func (s *SqlPostStore) Get(id string) (*model.PostList, *model.AppError) {
 
 	for _, p := range posts {
 		pl.AddPost(p)
+		pl.AddOrder(p.Id)
 	}
 
 	return pl, nil