Fix flyout nav on small viewports

What does this MR do?

CE port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25998

Decouples sidebar collapsed style from behavior.

More general info at https://gitlab.com/gitlab-org/gitlab-ce/issues/58727#note_149026347

More involved refactor at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25941

Screen_Recording_2019-03-11_at_18.28.33

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58727

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
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• Security reports checked/validated by reviewer