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/choose-account.html

@@ -14,7 +14,7 @@
           {{ heading }}
         </h2>
       </div>
-      <div class="column-three-quarters">
+      <div class="govuk-grid-column-three-quarters">
         <ul>
   {% else %}
     <ul>
@@ -62,7 +62,7 @@
             Platform admin
           </h2>
         </div>
-        <ul class="column-three-quarters">
+        <ul class="govuk-grid-column-three-quarters">
           <li class="browse-list-item">
             <a href="{{ url_for('.organisations') }}" class="govuk-link govuk-link--no-visited-state">All organisations</a>
             <p class="browse-list-hint">
@@ -106,7 +106,7 @@
           <div class="govuk-grid-column-one-quarter">
             &nbsp;
           </div>
-          <div class="column-three-quarters">
+          <div class="govuk-grid-column-three-quarters">
       {% endif %}
       {{ govukButton({
         "element": "a",