Fix up the excluded extensions bit.

This adds the missing admin class, and registration of it, required to
actually change the value of the excluded extensions configuration.

common/djangoapps/static_replace/admin.py

@@ -5,7 +5,7 @@ that gets prepended to asset URLs in order to serve them from, say, a CDN.
 from django.contrib import admin
 
 from config_models.admin import ConfigurationModelAdmin
-from .models import AssetBaseUrlConfig
+from .models import AssetBaseUrlConfig, AssetExcludedExtensionsConfig
 
 
 class AssetBaseUrlConfigAdmin(ConfigurationModelAdmin):
@@ -27,4 +27,24 @@ class AssetBaseUrlConfigAdmin(ConfigurationModelAdmin):
         return self.list_display
 
 
+class AssetExcludedExtensionsConfigAdmin(ConfigurationModelAdmin):
+    """
+    Basic configuration for asset base URL.
+    """
+    list_display = [
+        'excluded_extensions'
+    ]
+
+    def get_list_display(self, request):
+        """
+        Restore default list_display behavior.
+
+        ConfigurationModelAdmin overrides this, but in a way that doesn't
+        respect the ordering. This lets us customize it the usual Django admin
+        way.
+        """
+        return self.list_display
+
+
 admin.site.register(AssetBaseUrlConfig, AssetBaseUrlConfigAdmin)
+admin.site.register(AssetExcludedExtensionsConfig, AssetExcludedExtensionsConfigAdmin)