Implement user registration with reCAPTCHA

fixes #264

include/dialogs/ReCaptcha.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <QWidget>
+
+class FlatButton;
+class RaisedButton;
+
+namespace dialogs {
+
+class ReCaptcha : public QWidget
+{
+        Q_OBJECT
+
+public:
+        ReCaptcha(const QString &server, const QString &session, QWidget *parent = nullptr);
+
+protected:
+        void paintEvent(QPaintEvent *event) override;
+
+signals:
+        void closing();
+
+private:
+        FlatButton *openCaptchaBtn_;
+        RaisedButton *confirmBtn_;
+        RaisedButton *cancelBtn_;
+};
+} // dialogs