Include a cookie for a session id (experimental)

[david-yz-liu]

This task involves a couple of different parts:

1. Being able to set a cookie from the server from a request. See https://hackage.haskell.org/package/happstack-server-7.4.6.1/docs/Happstack-Server-Cookie.html. (Note: this should happen before every reponse, so it probably belongs in Server.hs.
2. Generate a random number to put in the cookie (see https://hackage.haskell.org/package/random-1.1/docs/System-Random.html)
3. Encrypt the random number in a cookie from the server (see https://hackage.haskell.org/package/clientsession-0.9.1.2/docs/Web-ClientSession.html)