Improve reporting upon moving/splitting topics

[neiljp]

Currently a message is given to the user when a topic is moved, but we could give more information about this:

• where from?
• where to?
• how many? (all? some?)

For the first version of this, updating the text we output would be sufficient.

As a follow-up, we could consider if there are other ways to indicate eg. split topics.

For comparison, the web app appears to intentionally not sort after splitting topics, to leave both parts in view, whereas ZT does so.

[srdeotarse]

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