ThreadListAvatars: add private thread indicator

When a thread belongs to a private forum, we can't obtain its avatars.
This changes makes this clear by inserting a "key" icon where the
avatars should be shown.

Bug: twpowertools:30
Change-Id: Idec33f277b12282df0fd271eebe0156865474bf4
diff --git a/src/static/css/thread_list_avatars.css b/src/static/css/thread_list_avatars.css
index 2dcb29b..beacbf1 100644
--- a/src/static/css/thread_list_avatars.css
+++ b/src/static/css/thread_list_avatars.css
@@ -6,19 +6,32 @@
   margin-inline-start: 14px;
 }
 
-.TWPT-avatars .TWPT-avatar {
+.TWPT-avatars .TWPT-avatar,
+    .TWPT-avatars .TWPT-avatar-private-placeholder {
   height: 28px;
   width: 28px;
   align-self: center;
   border-width: 0;
   border-radius: 50%;
   margin-inline-start: 6px;
+}
+
+.TWPT-avatars .TWPT-avatar {
   background-color: white;
   background-position: center;
   background-size: contain;
   background-repeat: no-repeat;
 }
 
+.TWPT-avatars .TWPT-avatar-private-placeholder {
+  line-height: 28px;
+  text-align: center;
+  color: #35363a;
+  background-color: #dadce0;
+  font-size: 20px;
+  font-family: 'Google Material Icons';
+}
+
 /*
  * Changing styles of existing elements so the avatars fit.
  */