Move Alert Detail view to new feature flag

What does this MR do?

Alert Management is being developed behind the alert_management_minimal feature flag. As part of enabling the feature (#216065 (closed)), we will remove this flag. However, the Detail view is not complete so we want to ensure this is still disabled. The solution for this is to give the detail view its own feature flag: alert_management_detail.

Behavior:

• Have an alert management alert loaded in the DB
• Navigate to <project-slug>/-/alert_management/<alert-iid>/details

Result:

• When alert_management_detail is enabled: See the alert detail page.
• When alert_management_detail is disabled: See the 404 page.

Issue: #216065 (closed)

No changelog required for this as it's currently behind a feature flag.

Screenshots

alert_management_detail enabled	alert_management_detail disabled

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation (if required)
• 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.
• [n/a] 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