WIP: Extract DiffDiscussionHeader component from NoteableDiscussion

What does this MR do?

This MR extracts DiffDiscussionHeader component from NoteableDiscussion. There are two possible code duplications: the firstNote computer property and toggleDiscussionHandler method. Both are quite short, so in my opinion it doesn't matter. Also there are not tests currently. Is the split done corretly?

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #58292 (moved)