Fix missing border for incident issues

What does this MR do?

The smallest MVC possible for: #326244 (closed) where we allow the bottom border to be drawn under milestones on incident issues to create alignment with issues.

This is done using a condition check on issuable_sidebar[:supports_iterations], and the reason we use this is that this evaluates to: !incident? which lets us target the incident details page leaving Epics, issues and merge details unaffected.

Screenshots (strongly suggested)

Issue	Merge	Incident

Does this MR meet the acceptance criteria?

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Related to #326244 (closed)