Draft: Show incidents via `/issues/incident/:iid`

What does this MR do?

PoC for #244943 (comment 415864473).

This MRs adds issues/incident/:iid so incidents issues can be viewed additionally via issues/incident/:iid.

Note, it's broken.

It's no longer broken but we need to:

TODO	Status
Fix To-Do Text
Fix sidebar naming from issue to incident i.e. "Move issue" => "Move incident"
Add feature flag
UX approval
Link Incidents to incident show from issue page
Linked Issues => Linked Incidents
Create project_incident_path
Update: spec/features/issues/incident_issue_spec.rb to use project_incident_path

/wip

Screenshots

Incident

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