Clean up logout procedure

In my hurry to fix #172 (closed), I left the logout code in suboptimal state. I'd like to

• Check what the hydra-returned post-logout URL is necessary for, and if needed find some way to use it.
• See if we can now remove the logout endpoint from the dashboard, because we already do the kratos logout in prelogout.
• If so, change the default post-logout url to something else, possibly having a message saying you're successfully logged out.
• Also end the dashboard session on logout. I think we did that but it doesn't work right now at least.
• General clean-up of the relevant dashboard code.