create SecureQLabel subclass of QLabel

[redshiftzero]

We should create a subclass of QLabel to help developers remember where they should be escaping html content, see:

securedrop-client/securedrop_client/gui/widgets.py

Line 1130 in 2c52b15

self.message = QLabel(html.escape(text, quote=False))

The SecureQLabel class should be used in all cases where user input is displayed in the UI - namely the message box (SpeechBubble ) and the preview snippet when implemented.

(suggested by this talk on leveraging type system for security and #160)