Avatars: show invisible icon when a thread isn't visible

When a thread isn't visible for anonymous users, we used to trigger an
error in the console. Now an icon is shown to users so they know avatars
couldn't be loaded due to the fact that the thread wasn't visible.

Fixed: twpowertools:105
Change-Id: If3808580ce322fe8e88ea45466725def277a60fc
diff --git a/src/static/_locales/en/messages.json b/src/static/_locales/en/messages.json
index f64219d..b78c8cc 100644
--- a/src/static/_locales/en/messages.json
+++ b/src/static/_locales/en/messages.json
@@ -283,6 +283,10 @@
     "message": "Due to technical reasons, we can't load the avatars of threads published in private forums.",
     "description": "Helper text which appears when hovering an icon next to a thread, to explain its meaning."
   },
+  "inject_threadlistavatars_invisible_thread_indicator_label": {
+    "message": "Due to technical reasons, we can't load the avatars of threads not visible to anonymous users.",
+    "description": "Helper text which appears when hovering an icon next to a thread, to explain its meaning."
+  },
   "inject_workflows_menubtn": {
     "message": "Run a workflow...",
     "description": "Tooltip of the icon shown above a thread or in thread lists when selecting multiple threads in the Community Console which lets the user show a menu with the worklofws they can run."