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/templates/withnav_template.html

@@ -31,7 +31,7 @@
       {% if help %}
         <div class="govuk-grid-column-two-thirds">
       {% else %}
-        <div class="column-three-quarters">
+        <div class="govuk-grid-column-three-quarters">
       {% endif %}
         {% block beforeContent %}{% endblock %}
         <main class="govuk-main-wrapper column-main {{ mainClasses }}" id="main-content" role="main">