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/manage-users/confirm-edit-user-email.html

@@ -15,7 +15,7 @@
 ) }}
 
 <div class="govuk-grid-row">
-  <div class="column-whole">
+  <div class="govuk-grid-column-full">
   {% call form_wrapper() %}
     <p>New email address:</p>
     <div class="panel panel-border-wide bottom-gutter">

app/templates/views/manage-users/confirm-edit-user-mobile-number.html

@@ -15,7 +15,7 @@
 ) }}
 
 <div class="govuk-grid-row">
-  <div class="column-whole">
+  <div class="govuk-grid-column-full">
   {% call form_wrapper() %}
     <p>New mobile number:</p>
     <div class="panel panel-border-wide bottom-gutter">

app/templates/views/manage-users/edit-user-mobile.html

@@ -17,7 +17,7 @@
 
   <p id="user_name">This will change the mobile number for {{ user.name }}.</p>
   <div class="govuk-grid-row">
-    <div class="column-three-quarters">
+    <div class="govuk-grid-column-three-quarters">
       {% call form_wrapper(class="extra-tracking") %}
         {{ textbox(form.mobile_number) }}
         {{ page_footer('Save') }}