Fix main-content-container to work with govuk-frontend columns

This commit removes some of the rules that were previously set for the
`.main-content-container` and instead uses the column rules from
`govuk-frontend`.

The container has now been renamed as `contents-container` to be
consistent with the `contents-list-container`. I had hoped to remove
this class entirely, but it's required to apply different float rules
when in RTL reading mode.

app/assets/stylesheets/views/_html-publication.scss

@@ -30,16 +30,7 @@
     }
   }
 
-  .main-content-container {
-    width: 100%;
-    padding: 0 15px;
-    box-sizing: border-box;
-
-    @include govuk-media-query($from: desktop) {
-      width: 75%;
-      float: right;
-    }
-
+  .contents-container {
     .direction-rtl & {
       float: left;
     }

app/views/content_items/html_publication.html.erb

@@ -64,7 +64,7 @@
       </div>
     </div>
 
-    <div class="main-content-container<% unless @content_item.contents.any? %> offset-empty-contents-list<% end %>">
+    <div class="govuk-grid-column-three-quarters-from-desktop contents-container<% unless @content_item.contents.any? %> offset-empty-contents-list<% end %>">
       <%= render "govuk_publishing_components/components/govspeak_html_publication", { direction: page_text_direction } do %>
         <%= raw(@content_item.govspeak_body[:content]) %>
       <% end %>