Menu button functionality (for issue #2)

To solve #2. 

- Added script in header.html to listen for events and handle them
- Added styles for the menu when in its new postion
- Changed some styles for nav--hidden so it animates nicely in new position
- Added styles for the mask
- Added a new close button in the nav menu
- Added styles for the close button
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- Still work in progress, needs checking at this stage