Add envClientSessionBackend, ENV-based session key

[pbrisbin]

This can be useful if:

1. You can't rely on a persistent file system (e.g. Heroku)
2. Your application is open source (e.g. you can't commit the key)

By keeping a consistent value in the environment variable, your users will have
consistent sessions without relying on the file system.

Usage:

makeSessionBackend _ = fmap Just $ envClientSessionBackend 120 "SESSION_KEY"

Related: #806

Totally open to bikeshedding the function name.

[gregwebs]

This looks great and it follows the existing pattern. It seems to me though that there is a problem with the getKey and getKeyEnv from clientsession: they both silently default to generating a new key when one is missing.

Do you think it would be a good idea to change these functions (actually to add new ones) that return an Either? Left would be a default, and in that case a warning can be printed.

[meteficha]

@gregwebs By "a default", you mean a predictable session key? I don't see how that would be an improvement.

[gregwebs]

@meteficha I opened up an issue in clientsession that hopefully explains it a little better. I am probably using the wrong terminology, I am not talking about changing any functionality, just exposing more information in the API.

yesodweb/clientsession#27