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/edit-email-template.html

@@ -18,7 +18,7 @@
 
     {% call form_wrapper() %}
       <div class="govuk-grid-row">
-        <div class="column-five-sixths">
+        <div class="govuk-grid-column-five-sixths">
           {{ textbox(form.name, width='1-1', hint='Your recipients will not see this', rows=10) }}
           {{ textbox(form.subject, width='1-1', highlight_placeholders=True, rows=2) }}
           {{ textbox(form.template_content, highlight_placeholders=True, width='1-1', rows=8) }}
@@ -29,7 +29,7 @@
             'Save'
           ) }}
         </div>
-        <aside class="column-whole">
+        <aside class="govuk-grid-column-full">
           {% include "partials/templates/guidance-formatting.html" %}
           {% include "partials/templates/guidance-personalisation.html" %}
           {% include "partials/templates/guidance-optional-content.html" %}