Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Authorization/MemberOwnerHandler.cs

@@ -0,0 +1,19 @@
+using System.Threading.Tasks;
+
+using Microsoft.AspNetCore.Authorization;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    public class MemberOwnerHandler: AuthorizationHandler<OwnSystemRequirement, PKMember> {
+        protected override Task HandleRequirementAsync(AuthorizationHandlerContext context,
+                                                       OwnSystemRequirement requirement, PKMember resource)
+        {
+            if (!context.User.Identity.IsAuthenticated) return Task.CompletedTask;
+            if (resource.System == context.User.CurrentSystem())
+                context.Succeed(requirement);
+            return Task.CompletedTask;
+        }
+    }
+}