Make `breakpoint-max-width` and `breakpoint-min-width` "public"

I have a case where I needed the raw pixel values from the
`$breakpoint-max-widths` variable. The `_breakpoint-max-width` and
`_breakpoint-min-width` helper methods already do this. By removing the
underscore prefix from these mixin methods, we make them "public". This
will help my case of centering a responsive 1-2-3 column-based layout (1
column on mobile, 2 columns on tablet, 3 columns on desktop):

  .container {
    margin: 0 auto;

    @include enhance(tablet) {
      max-width(breakpoint-min-width(tablet));
    }

    @include enhance(desktop) {
      max-width(breakpoint-min-width(desktop));
    }
  }

  .column-1 {
    float: left;

    @include enhance(tablet) {
      width: 70%;
    }

    @include enhance(desktop) {
      width: 60%;
    }
  }

  .column-2 {
    float: left;

    @include enhance(tablet) {
      width: 30%;
    }

    @include enhance(desktop) {
      width: 20%;
    }
  }

  .column-3 {
    float: left;

    @include enhance(tablet) {
      width: 30%;
    }

    @include enhance(desktop) {
      width: 20%;
    }
  }

Change-Id: If72d19f0c74fe78c83db335768a37fb28127b062
Reviewed-on: http://gerrit.causes.com/41198
Reviewed-by: Henric Trotzig <henric.trotzig@brigade.com>
Tested-by: Henric Trotzig <henric.trotzig@brigade.com>