DB: Updated handling of deleted user ID handling in DB

Updated uses of user ID to nullify on delete.
Added testing to cover deletion of user relations.
Added model factories to support changes and potential other tests.
Cleans existing ID references in the DB via migration.

database/factories/Activity/Models/ActivityFactory.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Database\Factories\Activity\Models;
+
+use BookStack\Activity\ActivityType;
+use BookStack\Users\Models\User;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/**
+ * @extends \Illuminate\Database\Eloquent\Factories\Factory<\BookStack\Activity\Models\Activity>
+ */
+class ActivityFactory extends Factory
+{
+    protected $model = \BookStack\Activity\Models\Activity::class;
+
+    /**
+     * Define the model's default state.
+     *
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        $activities = ActivityType::all();
+        $activity = $activities[array_rand($activities)];
+        return [
+            'type' => $activity,
+            'detail' => 'Activity detail for ' . $activity,
+            'user_id' => User::factory(),
+            'ip' => $this->faker->ipv4(),
+            'loggable_id' => null,
+            'loggable_type' => null,
+        ];
+    }
+}