Migrate API to ASP.NET Core Auth services + refactor

PluralKit.API/Controllers/v1/AccountController.cs

@@ -0,0 +1,33 @@
+using System.Threading.Tasks;
+
+using Microsoft.AspNetCore.Mvc;
+
+using Newtonsoft.Json.Linq;
+
+using PluralKit.Core;
+
+namespace PluralKit.API
+{
+    [ApiController]
+    [ApiVersion("1.0")]
+    [Route("a")]
+    [Route( "v{version:apiVersion}/a" )]
+    public class AccountController: ControllerBase
+    {
+        private IDataStore _data;
+
+        public AccountController(IDataStore data)
+        {
+            _data = data;
+        }
+
+        [HttpGet("{aid}")]
+        public async Task<ActionResult<JObject>> GetSystemByAccount(ulong aid)
+        {
+            var system = await _data.GetSystemByAccount(aid);
+            if (system == null) return NotFound("Account not found.");
+            
+            return Ok(system.ToJson(User.ContextFor(system)));
+        }
+    }
+}