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/options/experiments.html b/src/options/experiments.html
index a49e6c9..e25254b 100644
--- a/src/options/experiments.html
+++ b/src/options/experiments.html
@@ -12,6 +12,7 @@
       <h1 data-i18n="experiments_title"></h1>
       <p data-i18n="experiments_description"></p>
       <form>
+        <div class="option"><input type="checkbox" id="threadlistavatars"> <label for="threadlistavatars" data-i18n="threadlistavatars"></label></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>