Add Button for Maintainers to Rotate Instance Id

What does this MR do?

If an instance ID (used for authentication) is compromised or needs to
be recreated, a maintainer can now click on a button in the feature
flags configuration modal which will request a new one to be generated
and displayed.

Depends on gitlab-org/gitlab-ee!13432, gitlab-org/gitlab-ee!13450

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/11417

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team