Comments left in side-by-side mode on lines not part of diff become invisible in unified mode

[NikhilVerma]

Steps to reproduce

• Start review on a diff in side by side mode
• Leave a comment on a line not part of diff
• Finish review
• Pretend you are another developer and open review in unified mode

Possible Solutions

• Show review comments in unified mode even if their lines can't be found
• Don't allow adding review comments on lines which are not a part of the diff