Enable Notification Content extension on macOS 11+ (#1846)

Fixes #1592.

## Summary
Enables the Notification Content extension now that it works in Xcode 13.

## Any other notes
This is an entirely tooling enabling of this feature, as the extension works on macOS 11. There is a gotcha: the Notification Service Extension attachments are inaccessible. Additionally, macOS always displays the attached-by-us images/movies, which is a frustrating difference in behavior between macOS and iOS, but makes this particular deficiency less noticeable. Filed as FB9638431.

Sources/Extensions/NotificationContent/NotificationViewController.swift

@@ -63,8 +63,17 @@ class NotificationViewController: UIViewController, UNNotificationContentExtensi
         }
 
         // Try to grab the attachments, in case they failed or were lazy
+        let shouldDownload: Bool
 
-        if allowDownloads {
+        if Current.isCatalyst {
+            // catalyst doesn't have access to the system container for the builtin attachments
+            // however, it _also_ shows the system preview image in all cases, so we don't need to for that too
+            shouldDownload = attachmentURL == nil
+        } else {
+            shouldDownload = true
+        }
+
+        if allowDownloads, shouldDownload {
             return firstly {
                 Current.api
             }.then { api in