Notifications: Linked watch functionality to UI

Got watch system working to an initial base state.
Moved some existing logic where it makes sense.

app/Activity/Tools/UserWatchOptions.php

@@ -0,0 +1,98 @@
+<?php
+
+namespace BookStack\Activity\Tools;
+
+use BookStack\Activity\Models\Watch;
+use BookStack\Entities\Models\Entity;
+use BookStack\Users\Models\User;
+use Illuminate\Database\Eloquent\Builder;
+
+class UserWatchOptions
+{
+    protected static array $levelByName = [
+        'default' => -1,
+        'ignore' => 0,
+        'new' => 1,
+        'updates' => 2,
+        'comments' => 3,
+    ];
+
+    public function __construct(
+        protected User $user,
+    ) {
+    }
+
+    public function canWatch(): bool
+    {
+        return $this->user->can('receive-notifications') && !$this->user->isDefault();
+    }
+
+    public function getEntityWatchLevel(Entity $entity): string
+    {
+        $levelValue = $this->entityQuery($entity)->first(['level'])->level ?? -1;
+        return $this->levelValueToName($levelValue);
+    }
+
+    public function isWatching(Entity $entity): bool
+    {
+        return $this->entityQuery($entity)->exists();
+    }
+
+    public function updateEntityWatchLevel(Entity $entity, string $level): void
+    {
+        $levelValue = $this->levelNameToValue($level);
+        if ($levelValue < 0) {
+            $this->removeForEntity($entity);
+            return;
+        }
+
+        $this->updateForEntity($entity, $levelValue);
+    }
+
+    protected function updateForEntity(Entity $entity, int $levelValue): void
+    {
+        Watch::query()->updateOrCreate([
+            'watchable_id' => $entity->id,
+            'watchable_type' => $entity->getMorphClass(),
+            'user_id' => $this->user->id,
+        ], [
+            'level' => $levelValue,
+        ]);
+    }
+
+    protected function removeForEntity(Entity $entity): void
+    {
+        $this->entityQuery($entity)->delete();
+    }
+
+    protected function entityQuery(Entity $entity): Builder
+    {
+        return Watch::query()->where('watchable_id', '=', $entity->id)
+            ->where('watchable_type', '=', $entity->getMorphClass())
+            ->where('user_id', '=', $this->user->id);
+    }
+
+    /**
+     * @return string[]
+     */
+    public static function getAvailableLevelNames(): array
+    {
+        return array_keys(static::$levelByName);
+    }
+
+    protected static function levelNameToValue(string $level): int
+    {
+        return static::$levelByName[$level] ?? -1;
+    }
+
+    protected static function levelValueToName(int $level): string
+    {
+        foreach (static::$levelByName as $name => $value) {
+            if ($level === $value) {
+                return $name;
+            }
+        }
+
+        return 'default';
+    }
+}