sessionMode: Allow extensions at the login and unlock screens

Now extensions can specify which session modes they work in,
but specifying the login screen or unlock screen session modes in
an extensions metadata still won't work, because those session
modes disallow extensions.

This commit fixes that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1967>

js/ui/extensionSystem.js

@@ -74,7 +74,7 @@ var ExtensionManager = class {
     }
 
     _extensionSupportsSessionMode(uuid) {
-        let extension = this.lookup(uuid);
+        const extension = this.lookup(uuid);
 
         if (!extension)
             return false;

js/ui/sessionMode.js

@@ -42,6 +42,7 @@ const _modes = {
     },
 
     'gdm': {
+        allowExtensions: true,
         hasNotifications: true,
         isGreeter: true,
         isPrimary: true,
@@ -58,6 +59,7 @@ const _modes = {
     },
 
     'unlock-dialog': {
+        allowExtensions: true,
         isLocked: true,
         unlockDialog: undefined,
         components: ['polkitAgent', 'telepathyClient'],