Store: Fix missing invitation email when adding new user (#6372)

Fixes #6369, a regression introduced with#6188: Sending invite emails required a flag to be set, which defaulted to false. This fixes the code to explicitely set the flag and also defaults it to true (sending an invite email unless declared otherwise).

BTCPayServer/Controllers/UIStoresController.Users.cs

@@ -70,6 +70,7 @@ public partial class UIStoresController
                     Kind = UserRegisteredEventKind.Invite,
                     User = user,
                     InvitedByUser = currentUser,
+                    SendInvitationEmail = true,
                     CallbackUrlGenerated = tcs
                 });