Fix a phishing vulnerability related to HTML in memo fields

Original report by @s-rah here: ZcashFoundation#205

src/mainwindow.cpp

@@ -886,6 +886,7 @@ void MainWindow::setupTransactionsTab() {
 
         if (!memo.isEmpty()) {
             QMessageBox mb(QMessageBox::Information, tr("Memo"), memo, QMessageBox::Ok, this);
+            mb.setTextFormat(Qt::PlainText);
             mb.setTextInteractionFlags(Qt::TextSelectableByMouse | Qt::TextSelectableByKeyboard);
             mb.exec();
         }
@@ -934,6 +935,7 @@ void MainWindow::setupTransactionsTab() {
         if (!memo.isEmpty()) {
             menu.addAction(tr("View Memo"), [=] () {               
                 QMessageBox mb(QMessageBox::Information, tr("Memo"), memo, QMessageBox::Ok, this);
+                mb.setTextFormat(Qt::PlainText);
                 mb.setTextInteractionFlags(Qt::TextSelectableByMouse | Qt::TextSelectableByKeyboard);
                 mb.exec();
             });

src/txtablemodel.cpp

@@ -137,7 +137,7 @@ QVariant TxTableModel::data(const QModelIndex &index, int role) const {
                             return Settings::paymentURIPretty(Settings::parseURI(memo));
                         } else {
                             return modeldata->at(index.row()).type + 
-                            (memo.isEmpty() ? "" : " tx memo: \"" + memo + "\"");
+                            (memo.isEmpty() ? "" : " tx memo: \"" + memo.toHtmlEscaped() + "\"");
                         }
                     } else {
                         return concatMultipleMemos(dat);