Explitly set TextFormat to Plaintext for SecureQLabel

Presence of a `&lt` would result in the AutoText [1] to result in improper rendering of the Label, resulting in a segmentation fault (see #628). Setting the format to plaintext not only resolves this issue but also provides defense-in-depth to ensure RichText is not displayed.

[1]: https://doc.qt.io/qt-5/qt.html#TextFormat-enum

securedrop_client/gui/__init__.py

@@ -156,6 +156,7 @@ def __init__(
         flags: Union[Qt.WindowFlags, Qt.WindowType] = Qt.WindowFlags(),
     ):
         super().__init__(parent, flags)
+        self.setTextFormat(Qt.PlainText)
         self.setText(text)
 
     def setText(self, text: str) -> None: