Add migration wizard, fix migration from custom paste instance

- Very basic wizard just to allow the user to choose whether to keep
  their old paste settings or use the new default settings.

- People who used custom 0x0 instances would just be kept on those
  settings and won't see the wizard.

launcher/ui/setupwizard/PasteWizardPage.cpp

@@ -0,0 +1,42 @@
+#include "PasteWizardPage.h"
+#include "ui_PasteWizardPage.h"
+
+#include "Application.h"
+#include "net/PasteUpload.h"
+
+PasteWizardPage::PasteWizardPage(QWidget *parent) :
+    BaseWizardPage(parent),
+    ui(new Ui::PasteWizardPage)
+{
+    ui->setupUi(this);
+}
+
+PasteWizardPage::~PasteWizardPage()
+{
+    delete ui;
+}
+
+void PasteWizardPage::initializePage()
+{
+}
+
+bool PasteWizardPage::validatePage()
+{
+    auto s = APPLICATION->settings();
+    QString prevPasteURL = s->get("PastebinURL").toString();
+    s->reset("PastebinURL");
+    if (ui->previousSettingsRadioButton->isChecked())
+    {
+        bool usingDefaultBase = prevPasteURL == PasteUpload::PasteTypes.at(PasteUpload::PasteType::NullPointer).defaultBase;
+        s->set("PastebinType", PasteUpload::PasteType::NullPointer);
+        if (!usingDefaultBase)
+            s->set("PastebinCustomAPIBase", prevPasteURL);
+    }
+
+    return true;
+}
+
+void PasteWizardPage::retranslate()
+{
+    ui->retranslateUi(this);
+}