Add option to choose the thread page design in the CC

This feature allows users to choose whether to show the old thread page
design or the new one.

This CL also adds an experimental feature to activate the nested replies
experiment in the Community Console (only available in the Canary
channel). It has been added as an experimental feature because the
nested replies feature doesn't work yet in the Community Console.

Change-Id: Ifc371848b55f450bedf22d8e35fb4348fffa6c63
diff --git a/src/static/options/experiments.html b/src/static/options/experiments.html
index 9a21fdf..615d5ec 100644
--- a/src/static/options/experiments.html
+++ b/src/static/options/experiments.html
@@ -15,6 +15,7 @@
         <div id="optional-permissions-warning" hidden data-i18n="optionalpermissionswarning_header"></div>
         <div class="option"><input type="checkbox" id="workflows"> <label for="workflows" data-i18n="workflows"></label> <button id="manage-workflows" data-i18n="workflows_manage"></button></div>
         <div class="option"><input type="checkbox" id="extrainfo"> <label for="extrainfo" data-i18n="extrainfo"></label></div>
+        <div class="option"><input type="checkbox" id="nestedreplies"> <label for="nestedreplies" data-i18n="nestedreplies"></label></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>