Add threadListAvatars experiment

This experiment adds avatars of the users who have participated in a
thread in the thread list, next to each thread.

Change-Id: I259b103a7d3462201013ab2027866bbcce476901
diff --git a/src/common/common.js b/src/common/common.js
index 205cdc9..5703dbc 100644
--- a/src/common/common.js
+++ b/src/common/common.js
@@ -82,7 +82,10 @@
   },
 
   // Experiments:
-
+  'threadlistavatars': {
+    defaultValue: false,
+    context: 'experiments',
+  },
 
   // Internal options:
   'ccdarktheme_switch_enabled': {