Notifications: Started core user notification logic

Put together an initial notification.
Started logic to query and identify watchers.

app/Activity/Notifications/Handlers/PageCreationNotificationHandler.php

@@ -3,11 +3,32 @@
 namespace BookStack\Activity\Notifications\Handlers;
 
 use BookStack\Activity\Models\Loggable;
+use BookStack\Activity\Models\Watch;
+use BookStack\Activity\Tools\EntityWatchers;
+use BookStack\Activity\WatchLevels;
+use BookStack\Users\Models\User;
 
 class PageCreationNotificationHandler implements NotificationHandler
 {
-    public function handle(string $activityType, Loggable|string $detail): void
+    public function handle(string $activityType, Loggable|string $detail, User $user): void
     {
         // TODO
+
+        // No user-level preferences to care about here.
+        // Possible Scenarios:
+        // ✅ User watching parent chapter
+        // ✅ User watching parent book
+        // ❌ User ignoring parent book
+        // ❌ User ignoring parent chapter
+        // ❌ User watching parent book, ignoring chapter
+        // ✅ User watching parent book, watching chapter
+        // ❌ User ignoring parent book, ignoring chapter
+        // ✅ User ignoring parent book, watching chapter
+
+        // Get all relevant watchers
+        $watchers = new EntityWatchers($detail, WatchLevels::NEW);
+
+        // TODO - need to check entity visibility and receive-notifications permissions.
+        //   Maybe abstract this to a generic late-stage filter?
     }
 }