Notifications: Started core user notification logic

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

app/Activity/Notifications/Messages/BaseActivityNotification.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace BookStack\Activity\Notifications\Messages;
+
+use BookStack\Activity\Models\Loggable;
+use BookStack\Users\Models\User;
+use Illuminate\Bus\Queueable;
+use Illuminate\Notifications\Messages\MailMessage;
+use Illuminate\Notifications\Notification;
+
+abstract class BaseActivityNotification extends Notification
+{
+    use Queueable;
+
+    public function __construct(
+        protected Loggable|string $detail,
+        protected User $user,
+    ) {
+    }
+
+    /**
+     * Get the notification's delivery channels.
+     *
+     * @param  mixed  $notifiable
+     * @return array
+     */
+    public function via($notifiable)
+    {
+        return ['mail'];
+    }
+
+    /**
+     * Get the mail representation of the notification.
+     */
+    abstract public function toMail(mixed $notifiable): MailMessage;
+
+    /**
+     * Get the array representation of the notification.
+     *
+     * @param  mixed  $notifiable
+     * @return array
+     */
+    public function toArray($notifiable)
+    {
+        return [
+            'activity_detail' => $this->detail,
+            'activity_creator' => $this->user,
+        ];
+    }
+}