threadListAvatars: improve private thread indicator

Previously, the private thread indicator consisted in a "key" icon, and
according to one PE this led to confusion since it could be
misinterpreted as a lock icon.

This change replaces the icon by a "crossed person" icon, and adds a
tooltip to the icon which explains why that icon is shown.

Fixed: twpowertools:43
Change-Id: I2ec28114d93bf78be1b5f9999e65f916d7b2703f
diff --git a/src/static/css/thread_list_avatars.css b/src/static/css/thread_list_avatars.css
index beacbf1..2edf889 100644
--- a/src/static/css/thread_list_avatars.css
+++ b/src/static/css/thread_list_avatars.css
@@ -30,6 +30,7 @@
   background-color: #dadce0;
   font-size: 20px;
   font-family: 'Google Material Icons';
+  cursor: help;
 }
 
 /*