XL screen layout regression

[samajammin]

Describe the bug

We've had a regression of #207. Currently the site navigation stretches the width of the screen, regardless of width.

To Reproduce
Steps to reproduce the behavior:

1. Visit ethereum.org using an XL screen (i.e. width greater than 1440px)

Expected behavior

See #207 for context - site content (including the nav & footer) should not exceed a width of $contentWidthXL (1440px), no matter how wide the user's screen is. We should have a container that restricts this width.

Screenshots

[carlfairclough]

Huh. The offset there is surprising.

The footer is currently using the updated breakpoints, which are fluid with no currently defined max-width (see: https://www.figma.com/file/rQK2dt9eaLOMmvTOf5ifdr/Ethereum.org-Style-Guide-RC1?node-id=3%3A0).

The offset here, however, is wrong. I believe this fix should be part of #762

[samajammin]

Sounds good to me @carlfairclough. You game with setting a max width container? I don't know if that was ever discussed in the style guide.