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/views/service-settings/email_reply_to.html

@@ -40,7 +40,7 @@
     {% endfor %}
   </div>
   <div class="govuk-grid-row">
-    <div class="column-whole">
+    <div class="govuk-grid-column-full">
       {% if current_user.has_permissions('manage_service') %}
         <div class="js-stick-at-bottom-when-scrolling">
           {{ govukButton({
@@ -51,7 +51,7 @@
         </div>
       {% endif %}
     </div>
-    <div class="column-five-sixths">
+    <div class="govuk-grid-column-five-sixths">
       <p>
         You need to add at least one reply-to address so recipients can reply to your messages.
       </p>