Fix SubNav regressions

[rezrah]

Summary

Towards resolving release issues for SubNav

List of notable changes:

• Fixes z-index bug with global nav in dotcom
• Fixes mobile menu, which didn't work without aria-current. Restores that behaviour
• Updated default story to exclude aria-current
• Hides anchor navigation until it becomes sticky, per Site requirements (see slack thread)

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Latest commit: 5eb22d1

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