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/_locales/en/messages.json b/src/static/_locales/en/messages.json
index 2e0c9b4..a531f9a 100644
--- a/src/static/_locales/en/messages.json
+++ b/src/static/_locales/en/messages.json
@@ -250,5 +250,9 @@
   "inject_autorefresh_list_status_indicator_label_disabled": {
     "message": "Can't watch for updates to the thread list. Please change the sort options to 'Sort by: Last updated' and descending order.",
     "description": "Helper text which appears when hovering the status indicator of the 'autorefresh list' feature to indicate that the feature is disabled in the current thread list because the sort options are incompatible with the feature."
+  },
+  "inject_threadlistavatars_private_thread_indicator_label": {
+    "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."