Set site max width in pixels instead of ems

[adamjohnson]

Setting the site's max width in ems causes some issues if you change the font-size of a parent container—subsequently increasing the width of that block and messing alignment up in relation to other patterns.

This is noticable when you change the font size of the diptych and/or triptych patterns, most notably.

Fix this by overriding the default Neat variable in our variables file:

$max-width: 1088px;