Add sentry exception monitoring.

app.py

@@ -11,6 +11,7 @@ import features
 from storage import Storage
 from data.userfiles import Userfiles
 from util.analytics import Analytics
+from util.exceptionlog import Sentry
 from data.billing import Billing
 
 
@@ -44,3 +45,4 @@ storage = Storage(app)
 userfiles = Userfiles(app)
 analytics = Analytics(app)
 billing = Billing(app)
+sentry = Sentry(app)