Fixes alignment bug on content management empty state
Fixes alignment bug on content management empty state

When buttons changed to flexbox they lost some alignment properties. This change makes the container for the empty content state use a flexbox container and align its containing items correctly. Also removes some old media queries which are no longer needed.

kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin,dbalders/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin