Keep highlighted line when switching refs

What does this MR do and why?

This MR makes the highlighted line persist after you have switched the ref of a file.

Screenshots or screen recordings

Screencast_from_03.12.2021_22_40_35

How to set up and validate locally

1. Go to a project
2. Create a file with text content if not exists
3. Open the file
4. Edit the file & commit the changes to a different branch
5. Click on a line number
6. Switch the commit
7. See that you have still highlighted the same line number (not line necessarily when that line is different between the refs)

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.

Related to #323859 (closed)