BUG: Move topic needs to move subscriptions

[johnhenley]

Describe the bug

If you are subscribed to a topic, and the topic is moved to a different forum, the subscription looks like it has disappeared.

Software Versions

• DNN: 09.12.00
• Module: 07.00.12

To Reproduce

Steps to reproduce the behavior:

1. Subscribe to topic
   

2. Move topic to different forum

3. Subscription no longer exists
   

Expected behavior

Subscriptions should move with the topic.

Additional context

This is somewhat cosmetic once #535 is merged, which ignores the topic's forum when sending subscriptions to users who are subscribed to a specific topic. However, there are still some cosmetic/background changes that should be made to be consistent.