Fix #4990: Allow admins to delete uploads.

Allow admins to delete media asset files.

This only deletes the image file itself, not the upload or media asset record. The upload will still
be in the user's upload list, but the image will be gone. The media asset page will still exist, but
it will only show the file's metadata, not the image itself. We don't delete the metadata so we have
a record of what the file's MD5 was and who uploaded it, to prevent the file from being uploaded
again and to take action against the user if necessary.

test/functional/media_assets_controller_test.rb

@@ -44,5 +44,32 @@ class MediaAssetsControllerTest < ActionDispatch::IntegrationTest
         assert_response :success
       end
     end
+
+    context "destroy action" do
+      should "delete the asset's files" do
+        @admin = create(:admin_user)
+        @media_asset = MediaAsset.upload!("test/files/test.jpg")
+        delete_auth media_asset_path(@media_asset), @admin
+
+        assert_redirected_to @media_asset
+
+        assert_equal("deleted", @media_asset.reload.status)
+        @media_asset.variants.each do |variant|
+          assert_nil(variant.open_file)
+        end
+
+        assert_equal(1, ModAction.count)
+        assert_equal("media_asset_delete", ModAction.last.category)
+        assert_equal(@media_asset, ModAction.last.subject)
+        assert_equal(@admin, ModAction.last.creator)
+      end
+
+      should "fail for non-admins" do
+        @media_asset = create(:media_asset)
+        delete_auth media_asset_path(@media_asset), create(:user)
+
+        assert_response 403
+      end
+    end
   end
 end