Update common CSS for MR approvals

What does this MR do?

This contains the common css updates for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9003.

• Adds some common classes
• Updates common css layout (there's more to be done here)
• Adds classes for using to wrap inputs
• Adds select2 bootstrap validation support
• Fixes select2 in modal z-index
• Adds thead-white class

What are the relevant issue numbers?

• Part of gitlab-ee#1979
• FE gitlab-ee#8350

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer