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/static/css/ui_spacing/shared.css b/src/static/css/ui_spacing/shared.css
index c5b7cdd..2ae4fd8 100644
--- a/src/static/css/ui_spacing/shared.css
+++ b/src/static/css/ui_spacing/shared.css
@@ -27,7 +27,14 @@
   margin-bottom: 0!important;
 }
 
-.scTailwindThreadMessageMessagelistmessage-card {
+/**
+ * When flattening threads, we hide the reply button (in fact the entire blue
+ * footer), so we will reduce the padding moderately.
+ */
+body.TWPT-flattenthreads-enabled .scTailwindThreadMessageMessagelistmessage-card {
+  padding-bottom: 1rem!important;
+}
+body:not(.TWPT-flattenthreads-enabled) .scTailwindThreadMessageMessagelistmessage-card {
   padding-bottom: 0.75rem!important;
 }