Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Authorization/PrivacyRequirement.cs

@@ -0,0 +1,18 @@
+using System;
+
+using Microsoft.AspNetCore.Authorization;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    public class PrivacyRequirement<T>: IAuthorizationRequirement
+    {
+        public readonly Func<T, PrivacyLevel> Mapper;
+        
+        public PrivacyRequirement(Func<T, PrivacyLevel> mapper)
+        {
+            Mapper = mapper;
+        }
+    }
+}