Add hover state to todos

## What does this MR do?

- Add a hover state to todos like for repository files.
- Fix button alignment in todos
- Add link to avatar in todos

## Are there points in the code the reviewer needs to double check?

The `border-collapse` part—is there any better way?

## Why was this MR needed?

Make hover states consistent.

## What are the relevant issue numbers?

fixes #19833 

## Screenshots

### Before

![before](/uploads/cd2e7b234bf08fac48aebb60cdfa26a9/before.png)

### After

![todos-list](/uploads/592457050cb3ff2c43dfcce78418c50b/todos-list.png)

![todos-list-last-hover](/uploads/532714333bc74d1656a5c767012b4cdd/todos-list-last-hover.png)

![todos-mobile](/uploads/d542b04ddebc9d4354d57151dd04c0d2/todos-mobile.png)

See merge request !5361