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/styleguide.html

@@ -25,7 +25,7 @@
 
   <h2 class="heading-large">Banner</h2>
   <div class="govuk-grid-row">
-    <div class="column-three-quarters">
+    <div class="govuk-grid-column-three-quarters">
       <p>Used to show the status of a thing or action.</p>
 
       {{ banner("You sent 1,234 text messages", with_tick=True) }}
@@ -110,7 +110,7 @@
 
   <h2 class="heading-large">Tables</h2>
   <div class="govuk-grid-row">
-    <div class="column-three-quarters">
+    <div class="govuk-grid-column-three-quarters">
       <p>
         Used for comparing rows of data.
       </p>