Retain original topic title in shadow topic when moving a topic and editing the title.

If a topic's title is changed after it is moved (with shadow topic), the newly created topic shadow's subject remains the same, which appears in places such as the MCP. This can be confusing because of the discrepancy compared to viewtopic when a moderator wishes to remove a shadow topic.

When updating the topic (only when the subject is changed), topic_id needs to be compared against topic_moved_id as well.