review all breakpoints and containers in _custom-variables

[guylepage3]

Review all grid breakpoints and containers in _custom-variables.scss and adjust to new flexbox structure from custom spacing in previous blockstack-bootstrap``v4-alpha2

See https://github.com/blockstack/blockstack-bootstrap/issues/64

// 6. Grid breakpoints
//
// Define the minimum dimensions at which your layout will change,
// adapting to different screen sizes, for use in media queries.

$grid-breakpoints: (
  xs: 0,
  sm: 576px,
  md: 768px,
  lg: 992px,
  xl: 1200px
) !default;
@include _assert-ascending($grid-breakpoints, "$grid-breakpoints");
@include _assert-starts-at-zero($grid-breakpoints);

// 7. Grid containers
//
// Define the maximum width of `.container` for different screen sizes.

$container-max-widths: (
  sm: 540px,
  md: 720px,
  lg: 960px,
  xl: 1140px
) !default;
@include _assert-ascending($container-max-widths, "$container-max-widths");