main: Pick up overridesSchema from sessionMode

This will allow the use of mode-specific defaults. For classic mode
we currently implement this with mini-extensions, but this may result
in confusing behavior when settings change due to extensions being
disabled during screen locks (not to mention that those mini-extensions
are hardly an elegant approach).

https://bugzilla.gnome.org/show_bug.cgi?id=701717

js/ui/sessionMode.js

@@ -16,6 +16,7 @@ const _modes = {
     'restrictive': {
         parentMode: null,
         stylesheetName: 'gnome-shell.css',
+        overridesSchema: 'org.gnome.shell.overrides',
         hasOverview: false,
         showCalendarEvents: false,
         allowSettings: false,