Extend session periodically

[nicksellen]

I noticed I get logged out from time to time, even though I'm actively using karrot, and it's annoying!

I found django only extends the session when you modify it, which we don't, after the initial creation, so it just runs to expiry (2 weeks).

There is a django option to save on every request, but that's a bit much, so I created a middleware to extend the session every hour, if you're still making requests to the site.