Notifications: Started back-end for watch system

Added DB and started controller method.

app/Activity/Controllers/WatchController.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace BookStack\Activity\Controllers;
+
+use BookStack\Activity\Models\Watch;
+use BookStack\App\Model;
+use BookStack\Entities\Models\Entity;
+use BookStack\Http\Controller;
+use Exception;
+use Illuminate\Http\Request;
+use Illuminate\Validation\ValidationException;
+
+class WatchController extends Controller
+{
+    public function update(Request $request)
+    {
+        $requestData = $this->validate($request, [
+            'level' => ['required', 'string'],
+        ]);
+
+        $watchable = $this->getValidatedModelFromRequest($request);
+        $newLevel = Watch::optionNameToLevel($requestData['level']);
+
+        if ($newLevel < 0) {
+            // TODO - Delete
+        } else {
+            // TODO - Upsert
+        }
+    }
+
+    /**
+     * @throws ValidationException
+     * @throws Exception
+     */
+    protected function getValidatedModelFromRequest(Request $request): Entity
+    {
+        $modelInfo = $this->validate($request, [
+            'type' => ['required', 'string'],
+            'id'   => ['required', 'integer'],
+        ]);
+
+        if (!class_exists($modelInfo['type'])) {
+            throw new Exception('Model not found');
+        }
+
+        /** @var Model $model */
+        $model = new $modelInfo['type']();
+        if (!$model instanceof Entity) {
+            throw new Exception('Model not an entity');
+        }
+
+        $modelInstance = $model->newQuery()
+            ->where('id', '=', $modelInfo['id'])
+            ->first(['id', 'name', 'owned_by']);
+
+        $inaccessibleEntity = ($modelInstance instanceof Entity && !userCan('view', $modelInstance));
+        if (is_null($modelInstance) || $inaccessibleEntity) {
+            throw new Exception('Model instance not found');
+        }
+
+        return $modelInstance;
+    }
+}