Simplify implementation of homepage layout and styles (#6350)

- Removes reliance on bootstrap classes for layout and responsiveness.
- Stops using bootstrap-provided mixins and other utilities.
- Remove some unnecessary div nesting.
- Rely on same elements for narrow and wide layouts so that it's not
required to update two locations.
- Adds a missing missing aria attributes where necessary to improve
accessibility.
- Other general cleanup.

Contributes to #4164
Contributes to #5251

src/_layouts/homepage.html

@@ -5,10 +5,14 @@
     {% render 'cookie-notice.html' %}
     {% render 'gtags.html' %}
     {% include 'page-header.html' %}
-    {% include 'navigation-side.html' %}
-    <main id="page-content" class="focused">
-      {{content}}
-    </main>
-    {% render 'page-footer.html' %}
+    <div id="site-below-header">
+      <div id="site-main-row">
+        {% include 'navigation-side.html' %}
+        <main id="page-content" class="focused">
+          {{content}}
+        </main>
+      </div>
+      {% render 'page-footer.html' %}
+    </div>
   </body>
 </html>