WIP: Proof of concept for using Jest

What does this MR do?

This is a proof of concept for using Jest instead of Karma/Jasmine to run frontend tests.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/49171

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides