app.py: add some validation to set_username
app.py: add some validation to set_username

maniksingh92/flask-otr-webrtc-demo,spectralsun/flask-otr-webrtc-demo,spectralsun/flask-otr-webrtc-demo,foxichu/flask-otr-webrtc-demo,maniksingh92/flask-otr-webrtc-demo,spectralsun/flask-otr-webrtc-demo,maniksingh92/flask-otr-webrtc-demo,aldefalco/flask-otr-webrtc-demo,foxichu/flask-otr-webrtc-demo,aldefalco/flask-otr-webrtc-demo,aldefalco/flask-otr-webrtc-demo,foxichu/flask-otr-webrtc-demo