Moved the won message

On mobile, it really wasn't obvious that the sudoku was complete and
correct, so I put it in an overlay on the board.
Add close button to the won overlay.
Moved the status info

The info as to whether it's ok so far or not now shows as a table
overlay in xs mode. Not in anything else, as there is no real need, I
just thought it'd be nicer that way, then we can hide the buttons away
when not being used (not done yet) and just makes a much nicer mobile
interface.
Fixed the overlay jumping the board left

The board seemed to jump left when the overlay came on, due to some
weirdness. So I just stopped it being centred except on the xs view
(where it wasn't a problem, and was the only one that needed to be
centred).
Created board "pantry"

Created a "pantry" with a bunch of default boards to be used. They have
an ID, a board and a difficulty. Just now, it just takes the first.
Unchecking auto-pencil marks clears marks