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.

### 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 |
| ------ | ------ |
|  |  |
Update: The UI text should actually be **"Start another thread"** instead of "Start a new thread".
@jramsay @annabeldunstone @winh @brendan
issue