environment variable WBO_MAX_BOARD_SIZE_X to give max width of board (closes #118)

[droundy]

I just added a configuration variable MAX_BOARD_SIZE_X. I also made board.js obey the existing MAX_BOARD_SIZE (which it wasn't), and made it so you can't zoom out beyond the width of the board. The latter isn't technically necessary, and could in cases of a very wide window be annoying, but I think it's better than letting users confuse themselves with a bunch of white space that they can't write on.

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[lovasoa]

Can you have a look at the failing tests too ?

[droundy]

It looks like the failing tests assume a given transformation matrix, which is getting broken by my code that sets the minimum zoom level.

On second thought, it seems odd that the max scaling should be reached, unless either the window is very big in the test, or the max board size is small...

[lovasoa]

It looks like the cursor element (to which the transform should be applied) is not even present