Notifications: Started back-end for watch system

Added DB and started controller method.

app/Activity/Models/Watch.php

@@ -0,0 +1,52 @@
+<?php
+
+namespace BookStack\Activity\Models;
+
+use BookStack\Permissions\Models\JointPermission;
+use Carbon\Carbon;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+/**
+ * @property int $id
+ * @property int $user_id
+ * @property int $watchable_id
+ * @property string $watchable_type
+ * @property int $level
+ * @property Carbon $created_at
+ * @property Carbon $updated_at
+ */
+class Watch extends Model
+{
+    protected static array $levelByOption = [
+        'default' => -1,
+        'ignore' => 0,
+        'new' => 1,
+        'updates' => 2,
+        'comments' => 3,
+    ];
+
+    public function watchable()
+    {
+        $this->morphTo();
+    }
+
+    public function jointPermissions(): HasMany
+    {
+        return $this->hasMany(JointPermission::class, 'entity_id', 'watchable_id')
+            ->whereColumn('favourites.watchable_type', '=', 'joint_permissions.entity_type');
+    }
+
+    /**
+     * @return string[]
+     */
+    public static function getAvailableOptionNames(): array
+    {
+        return array_keys(static::$levelByOption);
+    }
+
+    public static function optionNameToLevel(string $option): int
+    {
+        return static::$levelByOption[$option] ?? -1;
+    }
+}