Unify collapse\expand all unresolved threads action

What does this MR do and why?

Implements Collapse all threads button doesn't fully colla... (#372020 - closed).

Collapse\expand all unresolved threads will always show or hide comments on the Changes tab in a merge request from now on.

This MR also changes the wording:

1. Collapse all unresolved threads → Hide all comments
2. Expand all unresolved threads → Show all comments

It also refactors the related discussions code to improve readability.

Screenshots or screen recordings

Before	After
Screen_Recording_2024-02-12_at_14.02.49	Screen_Recording_2024-02-12_at_14.00.42

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.