Add flattenthreads experiment

This experiment allows users to flatten the replies in threads, so they
are shown linearly in a chronological way instead of nested.

When the option is enabled, a switch is added to the thread page which
lets the user switch between flattening replies and not flattening them.

Some UI is still missing (see the design document[1]).

[1]: https://docs.google.com/document/d/1P-HanTHxaOFF_FHh0uSv0GIhG1dxWTJTGoT6VPjjvY0/edit

Bug: twpowertools:153
Change-Id: I43f94442cadc12b752700f0e8d974522be621d3e
diff --git a/src/xhrInterceptor/responseModifiers/index.js b/src/xhrInterceptor/responseModifiers/index.js
index 6a4573a..2d0b7ce 100644
--- a/src/xhrInterceptor/responseModifiers/index.js
+++ b/src/xhrInterceptor/responseModifiers/index.js
@@ -1,10 +1,12 @@
 import MWOptionsWatcherClient from '../../common/mainWorldOptionsWatcher/Client.js';
 import {convertJSONToResponse, getResponseJSON} from '../utils.js';
 
-import demo from './demo.js';
+import loadMoreThread from './loadMoreThread.js';
+import flattenThread from './flattenThread.js';
 
 export const responseModifiers = [
-  demo,
+  loadMoreThread,
+  flattenThread,
 ];
 
 // Content script target