Get view type based on diff_view Cookie

What does this MR do?

There was a bad checking when asking what type of diff view we were displaying currently.

Notes class was instantiated based on a value that came from the server (diff_view). The bug happened when you had two or more MR opened in multiple tabs and you changed the diff view to Side-by-side The other MRs were still trying to behave like the Notes were Inline. Check issue for details and steps to reproduce.

Are there points in the code the reviewer needs to double check?

Not really

Why was this MR needed?

To fix #27899 (closed)

Screenshots (if relevant)

Before. When it was reported

Right now you can't even add a new comment

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27899 (closed)