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

**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