UI for current user to move a topic between streams

[neiljp]

This issue represents the feature of moving a topic between streams, ie. not just renaming a topic inside the same stream.
This contrasts with #826, which is focused on updating the state from such changes for all other users via events.

Given that this currently requires admin+ rights, I've labelled this appropriately. We aren't currently focusing on admin actions, but if this becomes a moderator or stream-moderator feature in future then this may become higher priority.