Twenty Seventeen: Make fixed navigation apply at correct height on front page,...

Twenty Seventeen: Make fixed navigation apply at correct height on front page, without header video or image

Props laurelfulford, littlebigthing.

Fixes #38927.

Built from https://develop.svn.wordpress.org/trunk@39392


git-svn-id: https://core.svn.wordpress.org/trunk@39332 1a063a9b-81f0-0310-95a4-ce76da25c4cd