Fixed header being over issue boards in focus mode

Phil Hughesrequested to merge
issue-boards-focus-mode-fix into master

What does this MR do?

Fixes the z-index of the issue boards app when in focus mode. Previously the z-index was below the header which meant you couldn't toggle out of focus mode.

Screenshots (if relevant)

Screen_Shot_2017-05-30_at_14.36.10

What are the relevant issue numbers?

Closes #2473 (closed)

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