Fix jump to first discussion and cycle discussions

What does this MR do?

This MR changes the functionality of the "jump to first discussion" button to "jump to next discussion". The button now cycles through all unresolved discussions.

This MR also fixes a bug where unresolved discussions on older diffs could not be cycled through. So now the keyboard keys n & p and the next button will work on all unresolved discussions, not just the current diff 🎉

Related to: #16798 (closed) Related to: #28642 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content