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/ca/messages.json b/src/_locales/ca/messages.json
index 2c3ed92..9bf0e5c 100644
--- a/src/_locales/ca/messages.json
+++ b/src/_locales/ca/messages.json
@@ -99,6 +99,10 @@
     "message": "Posa el botó \"expandir fil\" a l'esquerra del tot en les llistes de fils de la Consola de la Comunitat.",
     "description": "Feature checkbox in the options page"
   },
+  "options_threadlistavatars": {
+    "message": "Mostra fotos de perfil a les llistes de fils de la Consola de la Comunitat.",
+    "description": "Feature checkbox in the options page"
+  },
   "options_profileindicator_header": {
     "message": "Punt indicador",
     "description": "Heading for the profile indicator feature options"