Upgrade to Bootstrap 4

What does this MR do?

Upgrade CE from Bootstrap 3 to 4 using migration checklist found in https://gitlab.com/gitlab-org/gitlab-ce/issues/45185#migration-checklist

How to test this MR

1. Check out branch
2. yarn install

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

Make sure everything is good enough to be merged into master. Bugs that can be fixed after being merged into master should have issues created with the label bootstrap4

We want to merge as soon as possible because each master merge/rebase increases the odds for UI regressions as we don't have a UI regression testing harness. It is also very tedious to keep fixing conflicts on a daily basis.

Why was this MR needed?

Prepare us for Bootstrap Vue and UI component library

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/45185, #46710 (closed)