Remove the edx.org comprehensive theme

There's a lot of useful things in PR #8271 that provide a framework
for the comprehensive theming system. If we need to remove the edx.org
theme from the codebase, we can leave most of PR #8271 in, to make it
easier to build on top of and get this feature back in.

cms/templates/widgets/header.html

@@ -12,7 +12,11 @@
 
     <div class="wrapper wrapper-l">
       <h1 class="branding"><a href="/">
-        <img src="${static.url("images/header-logo.png")}" alt="${settings.STUDIO_NAME}" />
+        % if settings.FEATURES.get('IS_EDX_DOMAIN', False):
+          <img src="${static.url("images/edx-theme/edx-studio-logo.png")}" alt="${settings.STUDIO_NAME}" />
+        % else:
+          <img src="${static.url("images/default-theme/logo.png")}" alt="${settings.STUDIO_NAME}" />
+        % endif
       </a></h1>
 
       % if context_course: