Adding a thread bubble is hidden when there is an existing comment on a line of code
## Proposal
Make it clear on how to add a thread versus viewing existing open comments in diffs
1. Reposition the add thread button to the area between the line numbers and +/-
1. Update avatar to a button to show comments per line

### Problem
Problem is that the button to add a thread is hidden when you go over a line with an existing comment.
> `@ms.mondrian` duo reviewer comment is blocking the UI to start the thread on the same line

### Designs
1. Mockups - [Walkthrough to leave feedback](https://gitlab.com/gitlab-org/gitlab/-/issues/573104/designs/1_-_start.png)
2. [Figma](https://www.figma.com/design/0ABBYYEvZTroTREb7yydtk/Comments-and-Diffs?node-id=52-83391&t=qANgP6E98O5O1uvw-1)
3. [Explorations](https://www.figma.com/design/0ABBYYEvZTroTREb7yydtk/Comments-and-Diffs?node-id=52-89178&t=qANgP6E98O5O1uvw-1)
4. [Prototype](https://www.figma.com/make/ADWuKnKfGHdC9E0jPoBjfp/Commenting-Prototype--Left-actions-?node-id=0-1&t=1MzAabt2w4X59Xxw-1)

issue