Implemented new discord linking system

Moonlight/App/Http/Controllers/Api/Moonlight/OAuth2Controller.cs

@@ -1,6 +1,7 @@
 using Logging.Net;
 using Microsoft.AspNetCore.Mvc;
 using Moonlight.App.Services;
+using Moonlight.App.Services.Sessions;
 
 namespace Moonlight.App.Http.Controllers.Api.Moonlight;
 
@@ -11,12 +12,41 @@ public class OAuth2Controller : Controller
     private readonly UserService UserService;
     private readonly OAuth2Service OAuth2Service;
     private readonly DateTimeService DateTimeService;
+    private readonly IdentityService IdentityService;
 
-    public OAuth2Controller(UserService userService, OAuth2Service oAuth2Service, DateTimeService dateTimeService)
+    public OAuth2Controller(
+        UserService userService,
+        OAuth2Service oAuth2Service,
+        DateTimeService dateTimeService,
+        IdentityService identityService)
     {
         UserService = userService;
         OAuth2Service = oAuth2Service;
         DateTimeService = dateTimeService;
+        IdentityService = identityService;
+    }
+
+    [HttpGet("{id}/start")]
+    public async Task<ActionResult> Start([FromRoute] string id)
+    {
+        try
+        {
+            if (OAuth2Service.Providers.ContainsKey(id))
+            {
+                return Redirect(await OAuth2Service.GetUrl(id));
+            }
+            
+            Logger.Warn($"Someone tried to start an oauth2 flow using the id '{id}' which is not registered");
+
+            return Redirect("/");
+        }
+        catch (Exception e)
+        {
+            Logger.Warn($"Error starting oauth2 flow for id: {id}");
+            Logger.Warn(e);
+            
+            return Redirect("/");
+        }
     }
 
     [HttpGet("{id}")]
@@ -24,6 +54,18 @@ public class OAuth2Controller : Controller
     {
         try
         {
+            var currentUser = await IdentityService.Get();
+
+            if (currentUser != null)
+            {
+                if (await OAuth2Service.CanBeLinked(id))
+                {
+                    await OAuth2Service.LinkToUser(id, currentUser, code);
+                    
+                    return Redirect("/profile");
+                }
+            }
+            
             var user = await OAuth2Service.HandleCode(id, code);
 
             Response.Cookies.Append("token", await UserService.GenerateToken(user), new()