Allow to rotate board with 90° steps

[jrmi]

To fix:

• The other player cursors are not updated on Zoom change
• The save modal can't be closed in some situations
• The generator fails sometimes
• The stacking feature doesn't always work in multiplayer situations
• Sending a big json through wire.io is not possible
• Use the center of an element for the stacking instead of the all content
• After holding item has moved, it is bring to front but not the child elements
• The select border displace the selected item