DRY beforeUnload behavior

[philippotto]

Description

See #5948 for context. The current behavior (which serves the browser's beforeUnload as well as the react-router's one) is currently used in:

• tracing layout view
• dataset import view
• dataset upload view

Ideally, we would use a react hook to DRY this mechanism, however, the using components would need to be converted to functional components first. Maybe a HOC is also a viable option.

Impact

Rather lower. The next time we touch this code, it would be a good idea to tackle this issue.

Cost

0.5 day ?