Adjust compose hints toward "general chat"#1425

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gnprice wants to merge 2 commits intozulip:mainfrom

gnprice:pr-compose-hint

Commits on Mar 20, 2025

compose [nfc]: Pull out method for computing topic for hint text
gnprice
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WIP compose: Omit "(no topic)" from hint when not allowed to send there; TODO fix tests, add tests
gnprice
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