[CSS] add scroll to live visits table #19273

Description:

Fixes: #19254
add scroll to live table

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

I've bumped the UI tests to run again, just want to be sure that the Movers and Shakers widget overflow is a random test failure and not something else.

The small screen dashboard UI test movers and shakers overflow is still failing each time on Travis but it passes when run locally, I've increase the delay from 250ms to 500ms to see if that fixes it.