Grid: Replace grid with CSS grid

[anselmbradford]

At some point we should migrate our grid to CSS grid to support model layout techniques https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout/Realizing_common_layouts_using_CSS_Grid_Layout#building_a_layout_using_the_12-column_system

[sonnakim]

We won't work on this —we'll be creating layouts using CSS grid and flexbox as we move towards using web components.

Some of this was done with the RTL work: layouts 2-1 and 1-3 were converted by Ans to use CSS grid. The only other template style we use extensively is the full layout. For this grid utility (as documented in the DS), Paying for College, Rate Explorer, and other one-off apps use the grid utility documented in the Design System, but Ans thinks doing layouts in this manner is brittle. As we move into a world where things are collections of components, the components themselves should handle their own layouts, so we'd want to think of grid utilities as deprecated — they're not intuitive to work with.

As we work on layout related issues, we should use flexbox or CSS grid, and minimize usage of the grid utility mixins.