app: session expiration - redirect to index when session expires

Previous logic when sessions expire on /generate:
1. On /generate page, codename is added to the session.
2. When expiration occurs, session is cleared.
3. A flashed message indicating that the session has expired
would be added to the current session (this is done via passing
`_flashes` on the session object [0]).
4. Execution enters the view function associated with `/create`.
But `/create` expects codename to be in the session (which was
cleared in step 2), thus a KeyError will occur.

Logic now when sessions expire on /generate:
1. On /generate page, codename is added to the session
2. When expiration occurs, session is cleared and user is redirected to the index.

[0] https://github.com/pallets/flask/blob/cd4023d9d2ab630ce4f95856f065072ef8badb2b/flask/helpers.py#L449

securedrop/source_app/__init__.py

@@ -145,7 +145,9 @@ def setup_g():
             # clear the session after we render the message so it's localized
             session.clear()
 
+            # Redirect to index with flashed message
             flash(Markup(msg), "important")
+            return redirect(url_for('main.index'))
 
         session['expires'] = datetime.utcnow() + \
             timedelta(minutes=getattr(config,