Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Authentication/AuthExt.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Security.Claims;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    public static class AuthExt
+    {
+        public static SystemId CurrentSystem(this ClaimsPrincipal user)
+        {
+            var claim = user.FindFirst(PKClaims.SystemId);
+            if (claim == null) throw new ArgumentException("User is unauthorized");
+            
+            if (int.TryParse(claim.Value, out var id))
+                return new SystemId(id);
+            throw new ArgumentException("User has non-integer system ID claim");
+        }
+        
+        public static LookupContext ContextFor(this ClaimsPrincipal user, PKSystem system)
+        {
+            if (!user.Identity.IsAuthenticated) return LookupContext.API;
+            return system.Id == user.CurrentSystem() ? LookupContext.ByOwner : LookupContext.API;
+        }
+
+        public static LookupContext ContextFor(this ClaimsPrincipal user, PKMember member)
+        {
+            if (!user.Identity.IsAuthenticated) return LookupContext.API;
+            return member.System == user.CurrentSystem() ? LookupContext.ByOwner : LookupContext.API;
+        }
+    }
+}