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/_locales/en/messages.json b/src/_locales/en/messages.json
index 7c8b45b..b8fbd5b 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -103,6 +103,10 @@
     "message": "Place the \"expand thread\" button all the way to the left in the Community Console thread lists.",
     "description": "Feature checkbox in the options page"
+  "options_threadlistavatars": {
+    "message": "Show avatars in thread lists in the Community Console.",
+    "description": "Feature checkbox in the options page"
+  },
   "options_profileindicator_header": {
     "message": "Indicator dot",
     "description": "Heading for the profile indicator feature options"