Working logout function

[jrjohnson]

When a user logs out they get redirected to the login page with a flash message confirming the logout.

Fixes #753

[saschaben]

can we move the login form down on the page about 60px, so the "login" label doesn't conflict with the flash message? With that and a code review this should be fine to push.

I think this is fine for the beta; we should probably review (with JenZ) to determine a better long term display model here; the flash message is a bit short for a message like this, probably want something just a little more persistent in this particular situation.

[jrjohnson]

The form is styled without anything special right now. I would prefer to merge and leave it that way and then take some time to completely restyle the experience later. (for example error messages are a bit off).

[saschaben]

That's fine, I'd still like to move it down the page 60px, as the overlay looks more like an accident right now, and less like a WIP. But if you are adamant, I'm OK. We'll just put it on the list for 0.5.0.