On-call Scheduling - Edit Rotation frontend

What does this MR do?

Adds the ability to edit the on-call schedule rotations.

Changes

• Activates the edit button for rotations.
• Adds behaviour for pre-populating / clearing the edit modal from the rotations list.
• Performs the edit mutation with populated data.

Follow-ups in the merge train

• Add rotation interval follow ups
  • Breaks out the form's $emit calls into a helper method.
  • Refactors the modal specs to use trigger instead of $emit.
• Add rotation length validation
• Rename rotation length

feature flag: :oncall_schedules_mvc

issue: https://gitlab.com/gitlab-org/gitlab/-/issues/262862

Testing

Note: Currently there is a bug where complexity limits are not properly applied 😿. This is under investigation. To work around this locally, navigate to app/graphql/gitlab_schema.rb and update the DEFAULT_MAX_COMPLEXITY field to 250 or above.

1. Enable the feature flag :oncall_schedules_mvc
2. Ensure you have a license of GitLab Premium or higher.
3. Navigate to Operations > On-call Schedules
4. Create a schedule.
5. Create a rotation.
6. Select the 'edit' button next to a rotation.
7. Edit the rotation and save. You should see your changes reflected in the UI.

Screenshots (strongly suggested)

before	after

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry Behind feature flag
• [n/a] Documentation (if required) Separate docs MR
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] 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
• [n/a] 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:

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