Notifications: added user preference UI & logic

Includes testing to cover.
Also added file missing from previous commit.

app/Activity/Notifications/NotificationManager.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace BookStack\Activity\Notifications;
+
+use BookStack\Activity\ActivityType;
+use BookStack\Activity\Models\Loggable;
+use BookStack\Activity\Notifications\Handlers\CommentCreationNotificationHandler;
+use BookStack\Activity\Notifications\Handlers\NotificationHandler;
+use BookStack\Activity\Notifications\Handlers\PageCreationNotificationHandler;
+use BookStack\Activity\Notifications\Handlers\PageUpdateNotificationHandler;
+
+class NotificationManager
+{
+    /**
+     * @var class-string<NotificationHandler>[]
+     */
+    protected array $handlers = [];
+
+    public function handle(string $activityType, string|Loggable $detail): void
+    {
+        $handlersToRun = $this->handlers[$activityType] ?? [];
+        foreach ($handlersToRun as $handlerClass) {
+            /** @var NotificationHandler $handler */
+            $handler = app()->make($handlerClass);
+            $handler->handle($activityType, $detail);
+        }
+    }
+
+    /**
+     * @param class-string<NotificationHandler> $handlerClass
+     */
+    public function registerHandler(string $activityType, string $handlerClass): void
+    {
+        if (!isset($this->handlers[$activityType])) {
+            $this->handlers[$activityType] = [];
+        }
+
+        if (!in_array($handlerClass, $this->handlers[$activityType])) {
+            $this->handlers[$activityType][] = $handlerClass;
+        }
+    }
+
+    public function loadDefaultHandlers(): void
+    {
+        $this->registerHandler(ActivityType::PAGE_CREATE, PageCreationNotificationHandler::class);
+        $this->registerHandler(ActivityType::PAGE_UPDATE, PageUpdateNotificationHandler::class);
+        $this->registerHandler(ActivityType::COMMENT_CREATE, CommentCreationNotificationHandler::class);
+    }
+}