Twitch API: v5 to Helix migration (#1560)

There's a document in src/providers/twitch/api which describes how we interact with the Twitch API.
Keeping this up to date might be a healthy way for us to ensure we keep using the right APIs for the right job.

src/widgets/dialogs/LoginDialog.hpp

@@ -57,12 +57,6 @@ public:
             QPushButton addUserButton;
             QPushButton clearFieldsButton;
         } buttonUpperRow;
-
-        struct {
-            QHBoxLayout layout;
-
-            QPushButton fillInUserIDButton;
-        } buttonLowerRow;
     } ui_;
 };