Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Authorization/SystemPrivacyHandler.cs

@@ -0,0 +1,21 @@
+using System.Threading.Tasks;
+
+using Microsoft.AspNetCore.Authorization;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    public class SystemPrivacyHandler: AuthorizationHandler<PrivacyRequirement<PKSystem>, PKSystem>
+    {
+        protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
+                                                       PrivacyRequirement<PKSystem> requirement, PKSystem resource)
+        {
+            var level = requirement.Mapper(resource);
+            var ctx = context.User.ContextFor(resource);
+            if (level.CanAccess(ctx))
+                context.Succeed(requirement);
+            return Task.CompletedTask;
+        }
+    }
+}