Factorize breakpoint map handling in grid gutters and margins

Factorize breakpoint map handling in `grid-column-gutter` and
`grid-column-margin` mixins.

Changes:
- Add the `-zf-breakpoint-value` mixin which generate its content with
the value `$-zf-bp-value` depending on its parameter, like
`grid-column-gutter` and `grid-column-margin` did (see the `@mixin
-zf-breakpoint-value` doc)
- Use the `-zf-breakpoint-value` mixin in `grid-column-gutter` and
`grid-column-margin` and remove duplicated code.
- Rename the `grid-column-gutter` function to `-zf-breakpoint-value`
and move it to `/scss/util/_breakpoint.scss`. This function had a
generic behaviour.