Focus mode of issues board hides scrollbar

Focus mode of issues board hides both the header and the horizontal scrollbar.
So in this mode the boards are unusable if there are multiple columns shown that don't fit to the screen.

Focus mode off	Focus mode on

@victorwu, can we take a look at gitlab-ce#22933 again?

I'm sure it would solve this problem and make UX of issue boards better.

Proposal:

Show the horizontal scrollbar when in focus mode.