Add feature flag for hiding edit rotations [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

We're approaching being able to test on-call schedule management. To make it easier to provide a seamless experience for testing, we should hide the ability to edit rotations behind a separate flag, since it won't be 100% ready for smoke testing next week.

This MR adds a flag and makes it available to the frontend. We should remove this flag at the same time as the oncall_schedules_mvc flag.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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