Port Configure Feature Flags Modal to Vue

What does this MR do?

An interactive vue version is required for rotating the instance ID token.

This ports the existing haml modal as-is to vue, with work adding the rotating to come later.

Depends on gitlab-org/gitlab-ce!28676 for a copy button that works within vue modals.

Parrt of work towards gitlab-org/gitlab-ee#11417.

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers