Fixed hidden truncated diff lines not showing when expanded

What does this MR do?

Fixes collapsed discussions diff lines not showing when expanded. This is because the truncated_diff_lines property does not exist on the discussion when it gets expanded and is therefore not reactive. To make it reactive it gets added when the discussions are loaded.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides