Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Authorization/SystemOwnerHandler.cs

@@ -0,0 +1,20 @@
+using System.Threading.Tasks;
+
+using Microsoft.AspNetCore.Authorization;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    public class SystemOwnerHandler: AuthorizationHandler<OwnSystemRequirement, PKSystem>
+    {
+        protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
+                                                       OwnSystemRequirement requirement, PKSystem resource)
+        {
+            if (!context.User.Identity.IsAuthenticated) return Task.CompletedTask;
+            if (resource.Id == context.User.CurrentSystem()) 
+                context.Succeed(requirement);
+            return Task.CompletedTask;
+        }
+    }
+}