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/contentScripts/communityConsole/avatars.js b/src/contentScripts/communityConsole/avatars.js
index 093e217..437e68c 100644
--- a/src/contentScripts/communityConsole/avatars.js
+++ b/src/contentScripts/communityConsole/avatars.js
@@ -316,7 +316,10 @@
           if (res.state == 'private') {
             var avatar = document.createElement('div');
             avatar.classList.add('TWPT-avatar-private-placeholder');
-            avatar.textContent = 'vpn_key';
+            avatar.textContent = 'person_off';
+            var label = chrome.i18n.getMessage(
+                'inject_threadlistavatars_private_thread_indicator_label');
+            avatar.setAttribute('title', label);
             avatarsContainer.appendChild(avatar);
           } else {
             for (var i = 0; i < avatarUrls.length; ++i) {