Updating site to use USWDS styles (#509)

* Updated header and footer
* Updated fonts
* Moved files around and updated gulpfile to correct the build process when it goes to production
* Adjusted grid templating
* Added images to assets
* Update app/templates/components/uk_components/footer/template.njk

Co-authored-by: Steven Reilly <stvnrlly@users.noreply.github.com>

app/templates/views/platform-admin/get-billing-report.html

@@ -8,7 +8,7 @@
 
 {% block platform_admin_content %}
 
-  <h1 class="heading-large">
+  <h1 class="font-body-2xl margin-bottom-3">
     Billing Report
   </h1>
 
@@ -18,7 +18,7 @@
     {{ page_footer('Download report (CSV)') }}
   {% endcall %}
 
-  <h2 class="heading-medium">
+  <h2 class="font-body-lg">
     Data included in the report
   </h2>
 <div class="bottom-gutter-3-2">