Replace mega hack with mini hack.

[notlee]

Makes the aside-sidebar of the query layout optional.

I think this is better for users than the wrapper approach and the previous modifier approach because it requires no additional markup. And we get to remove the _getMainMaxWidth mega hack 🎉

Unrelated this PR also corrects footer markup, and updates the migration guide accordingly.

Good

• Simplifies the SCSS (removes _getMainMaxWidth super grossness).
• Makes the css grid more flexible as the main content can be expressed in fractions rather than px.
• To not use a sidebar on the query layout, just don't include the sidebar element.
• Easy to document and understand.

Bad

• The grid gutter for the sidebar takes up space even when the sidebar is empty.

[notlee]

There's no mega hack but there is a mini hack 😅 I'll add code comments on Monday
left: #{calc((100vw - 100%) * -0.5)};

[notlee]

Our modivation is to support registry-like layouts and layouts like that pioneered by bizops.

[gvonkoss]

I may have missed something in one of the last PRs, but I have question 👇 and some smol comments ✌️

I like the non modifier non wrapper business though!

[gvonkoss]

So this is happening because the grid doesn't span the whole page, right? Why did we remove the five columns again?

[notlee]

Yeah, it's to remove the mega hack which calculated in SCSS a px value for the main content area. We either need a wrapper around the main content and sidebar, or we set the whole of .o-layout to the max width of the container and then make the header and footer "break out" of the container (which is what this is).

[notlee]

Ah, and the mega hack is bad because if the main content area is set in px it can't expand when the sidebar is deleted.