Improve design for multiple discussions per line in merge request diffs

Problem to solve

The initial proposal for #32452 (closed) created confusion around why there are two boxes underneath each other where users can either reply or start a new thread.

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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. I have quickly put together two versions.

The first one is the most neutral, cleaner, and currently my preferred version. The second one highlights that this new thread will also appear for this line, which might help with understanding this new functionality.

One thread Three threads
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@jramsay @annabeldunstone @winh @brendan

Edited by Marcel van Remmerden