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/find-users/find-users-by-email.html

@@ -18,7 +18,7 @@
       action=url_for('.find_users_by_email'),
       class='govuk-grid-row'
   ) %}
-    <div class="column-three-quarters">
+    <div class="govuk-grid-column-three-quarters">
       {{ textbox(
         form.search,
         width='1-1',

app/templates/views/find-users/user-information.html

@@ -7,7 +7,7 @@
 
 {% block platform_admin_content %}
   <div class="govuk-grid-row bottom-gutter">
-    <div class="column-whole">
+    <div class="govuk-grid-column-full">
       <h1 class="heading-large">
         {{ user.name }}
       </h1>