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/api/callbacks/delivery-status-callback.html

@@ -10,7 +10,7 @@
 
 {% block maincolumn_content %}
   <div class="govuk-grid-row">
-    <div class="column-five-sixths">
+    <div class="govuk-grid-column-five-sixths">
 
       {{ page_header(
         'Callbacks for delivery receipts',

app/templates/views/api/callbacks/received-text-messages-callback.html

@@ -14,7 +14,7 @@
     back_link=url_for('.api_callbacks', service_id=current_service.id)
   ) }}
   <div class="govuk-grid-row">
-    <div class="column-five-sixths">
+    <div class="govuk-grid-column-five-sixths">
       <p>
         When you receive a text message in Notify, we can forward it to your system.
         Check the <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.callbacks') }}"> callback documentation </a> for more information.