lightbox: Use Zulip copy icon for "Copy link" button

Replace the generic Icons.copy with ZulipIcons.copy for the
"Copy link" button in the lightbox UI. This matches the icon
used in the message action sheet's "Copy message text" button
and follows our Figma designs.

Fixes: #2014

Author: sarafarajnasardi

lib/widgets/lightbox.dart

@@ -16,6 +16,7 @@ import 'message_list.dart';
 import 'page.dart';
 import 'store.dart';
 import 'user.dart';
+import 'icons.dart';
 
 /// Identifies which [LightboxHero]s should match up with each other
 /// to produce a hero animation.
@@ -104,7 +105,7 @@ class _CopyLinkButton extends StatelessWidget {
     final zulipLocalizations = ZulipLocalizations.of(context);
     return IconButton(
       tooltip: zulipLocalizations.lightboxCopyLinkTooltip,
-      icon: const Icon(Icons.copy),
+      icon: const Icon(ZulipIcons.copy),
       onPressed: () async {
         PlatformActions.copyWithPopup(context: context,
           successContent: Text(zulipLocalizations.successLinkCopied),