Fix notifications issues

[joshheald]

Closes: #

Description

In #12443, in-app notifications (when the app is in the foreground) were inadvertently broken, and so was navigation/handling of background notifications.

This was because we removed the delegate link to the UNUserNotificationCenter, so the delegate methods AppDelegate.userNotificationCenter(_:didReceive:) and AppDelegate.userNotificationCenter(_:willPresent:) were never called, and so not forwarded to the pushNotesManager.

This commit restores the delegate configuration.

Testing instructions

1. launch the app
2. select a store connected via jetpack, and leave the app open
3. on another device, make a purchase for anything
4. The app should show a notice saying you've got a new order, with a view button...

Repeat the test with the app in the background. Tapping the notification should open the order.

---

• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[dangermattic]

	1 Warning
:warning:	This PR is assigned to the milestone 18.4. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

Generated by :no_entry_sign: Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr12633-0a604d2
	Version	18.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	0a604d2b60f491b04a63d3c797c50a065db95ec1
	App Center Build	WooCommerce - Prototype Builds #8928

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