status: Rebrand the user menu as the system menu

As the user menu no longer really shows anything about the user, and just
power menu and settings, it's not really deserving of the "user menu" name,
so rename it to the ever-blandly-named "system menu".

This is a part of the new system status design, see
https://wiki.gnome.org/GnomeShell/Design/Guidelines/SystemStatus/
for design details.

https://bugzilla.gnome.org/show_bug.cgi?id=704368

js/ui/sessionMode.js

@@ -60,7 +60,7 @@ const _modes = {
         unlockDialog: undefined,
         components: ['polkitAgent', 'telepathyClient'],
         panel: {
-            left: ['userMenu'],
+            left: [],
             center: [],
             right: ['lockScreen']
         },
@@ -72,7 +72,7 @@ const _modes = {
         unlockDialog: undefined,
         components: ['polkitAgent', 'telepathyClient'],
         panel: {
-            left: ['userMenu'],
+            left: [],
             center: [],
             right: ['a11y', 'keyboard', 'lockScreen']
         },
@@ -97,7 +97,7 @@ const _modes = {
             left: ['activities', 'appMenu'],
             center: ['dateMenu'],
             right: ['a11y', 'keyboard', 'volume', 'bluetooth',
-                    'network', 'battery', 'userMenu']
+                    'network', 'battery', 'system']
         }
     }
 };