Standardize grid gutters

Standardize grid gutters functions, mixins and arguments.

Adds/Removes:
- Add `grid-column-gutter` function: Get a gutter size for a given
breakpoint.
- Allow `grid-column-size` mixin to take breakpoint name as argument
- Remove `grid-column-uncollapse` mixin. Use `grid-column-size` instead.
- Use these new features in all `(flex-)grid` mixin & functions. Use
`grid-column-gutter` function instead of modify `$grid-column-gutter`
variable

Minor changes:
- Use `$gutter` for a single size, and `$gutters` for a size map