Replace the .column-... classes in _grids.scss

We had 7 classes in _grids.scss named `.column-...` which were being
used to give a certain column width. These worked by using `@include
grid column()`, which is now deprecated.

`.column-whole` and `.column-three-quarters` can be removed and replaced
with `govuk-grid-column-full` and `govuk-grid-column-three-quarters`
respectively. The other column classes don't have a direct replacment in
GOV.UK Frontend. To get round this, we overwrite the `$govuk-grid-width`
SASS map in `extensions.scss` to add in extra widths, then use this with
the `govuk-grid-column` mixin to create new classes in for our custom
widths in `_grids.scss`

app/assets/stylesheets/govuk-frontend/extensions.scss

@@ -35,3 +35,16 @@
 .govuk-link--destructive {
   @include govuk-link-style-destructive-no-visited-state;
 }
+$govuk-grid-widths: (
+  one-eighth: 12.5%,
+  one-sixth: 16.6666%,
+  one-quarter: 25%,
+  one-third: 33.3333%,
+  one-half: 50%,
+  five-eighths: 62.5%,
+  two-thirds: 66.6666%,
+  three-quarters: 75%,
+  five-sixths: 83.3333%,
+  seven-eighths: 87.5%,
+  full: 100%
+);