Displays change that made a diff line outdated

What does this MR do and why?

Gives users a button inside of the changed this line system note that allows them to view the change that made the discussion outdated.

Screenshots or screen recordings

Screen_Recording_2021-10-27_at_16.18.37

Screen_Recording_2021-10-27_at_16.19.04

Screen_Recording_2021-10-27_at_16.19.16

How to set up and validate locally

1. Enable the display_outdated_line_diff feature flag
2. Create a new merge request with some changes
3. Create a discussion on a changed line
4. Push a change that changes the same line that the discussion is on
5. View the toggle in the changed this line system note

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR. Closes #24468 (closed)