Fix channel_deleted post type (#4841)

model/post.go

@@ -118,7 +118,9 @@ func (o *Post) IsValid() *AppError {
 	}
 
 	// should be removed once more message types are supported
-	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE || o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE || o.Type == POST_DISPLAYNAME_CHANGE) {
+	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE ||
+		o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE ||
+		o.Type == POST_DISPLAYNAME_CHANGE || o.Type == POST_CHANNEL_DELETED) {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type)
 	}