Improve design for multiple discussions per line in merge request diffs
### Problem to solve The initial proposal for gitlab-ce#32452 created confusion around why there are two boxes underneath each other where users can either reply or start a new thread. ![image](/uploads/667ffe70a76fc9a87ab812cb77da63eb/image.png) ### Proposal In order to make `Reply` and `Start a new thread` more visually distinctive, we could switch out the bottom avatar + input field pattern for a button. | One thread | Three threads | | ------ | ------ | | ![image](/uploads/ed3768c799893e660e7b75ba6d80e0de/image.png) | ![image](/uploads/c962e892171afc9cdf10dc28d2ab1ab8/image.png) | Update: The UI text should actually be **"Start another thread"** instead of "Start a new thread". @jramsay @annabeldunstone @winh @brendan
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