Unify Fido2 authentication under two-factor tab (#2866)

* Unify Fido2 authentication under two-factor tab

Closes #2754.

* Improve UI and wording

* Improve register FIDO2 device page

BTCPayServer/Controllers/ManageController.2FA.cs

@@ -29,6 +29,7 @@ namespace BTCPayServer.Controllers
             {
                 Is2faEnabled = user.TwoFactorEnabled,
                 RecoveryCodesLeft = await _userManager.CountRecoveryCodesAsync(user),
+                Credentials = await _fido2Service.GetCredentials( _userManager.GetUserId(User))
             };
 
             return View(model);