GH-4071 Heavily refactor and rearchitect account system

This makes the account system much more modular
and makes it treat errors as something recoverable,
unless they come directly from the MSA refresh token
becoming invalid.

launcher/minecraft/auth/flows/Mojang.h

@@ -0,0 +1,26 @@
+#pragma once
+#include "AuthFlow.h"
+
+class MojangRefresh : public AuthFlow
+{
+    Q_OBJECT
+public:
+    explicit MojangRefresh(
+        AccountData *data,
+        QObject *parent = 0
+    );
+};
+
+class MojangLogin : public AuthFlow
+{
+    Q_OBJECT
+public:
+    explicit MojangLogin(
+        AccountData *data,
+        QString password,
+        QObject *parent = 0
+    );
+
+private:
+    QString m_password;
+};