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/contentScripts/communityConsole/threadToolbar/constants.js b/src/contentScripts/communityConsole/threadToolbar/constants.js
new file mode 100644
index 0000000..756d880
--- /dev/null
+++ b/src/contentScripts/communityConsole/threadToolbar/constants.js
@@ -0,0 +1,5 @@
+export const kEventFlattenThreadsUpdated =
+    'TWPTThreadToolbarFlattenThreadsUpdated';
+
+export const kRepliesSectionSelector =
+    'ec-thread .scTailwindThreadThreadcontentreplies-section';