Add incident details flag

What does this MR do?

A small MR to hide the dedicated Incident details page behind a feature flag

This merge is the second of sex merge requests that will allow us to create a base dedicated incident page with a stripped down sidebar.

• Create Incident Details Scaffold
• Create feature flag to hide details route
• Add sidebar scaffold to details page
• Add link from current Incident List to issue details
• Remove Epic feature from Incidents
• Scaffold Incident details layout => Errors, Title, description, details, base metrics

For the moment, this MVC uses the Issues API and will transferred to the Incident API once that is completed.

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