add plaintext to replybox placeholder

support for rich text is disabled for security purposes, which
means we will have to find another way to have the source name in
bold while at the same time preventing the user from inputing rich
text

securedrop_client/gui/widgets.py

@@ -2290,6 +2290,7 @@ def __init__(self, source: Source, controller: Controller) -> None:
 
         # Create relybox widgets
         self.text_edit = QPlainTextEdit()
+        self.text_edit.setPlaceholderText("Compose a reply to %bold_source_name%")
 
         self.send_button = QPushButton()
         self.send_button.clicked.connect(self.send_reply)