Make sessions expire sooner than default period

According to [this StackOverflow post](https://stackoverflow.com/questions/11783025/is-there-an-easy-way-to-make-sessions-timeout-in-flask), sessions expire in 31 days by default. I have a logout timer, but there should be some server-side failsafe as well. That's why sessions should expire after some sane value (ex. 1 hour).
