Persist session ID to identify owners of anonymously created compositions

[cheshire137]

When I check the Application > Cookies view in Chrome's web inspector for the app, I see a session cookie from Rails whose expiry time is "Session." "Session" cookies are handled differently by browser, and Chrome will keep your session cookies around, as I understand it, if the user has selected the "Continue where you left off" Chrome browser setting. I don't think we want to depend on this, as per RQ06, to hook up a bunch of ownerless compositions records if a user who has been using the site anonymously decides to sign up.

http://stackoverflow.com/a/1232231 looks relevant for modifying how long a Rails session cookie lasts. I'm not sure if that will be a rolling expiration date, like if we set it for a year in the future and the user uses the app again after 364 days, will the new expiration date be another year beyond that?