Admin Panel: In-context re-login

[HeyJoel]

When logged out of the admin panel (specifically while editing), it would be good to be able to do an in-context login modal so that data isn't lost by the login redirection.

Original text:

Even worse is that this website has such a short timeout that I cannot even complete a complex text before I'm logged out - and then I loose all the stuff I entered when I click "Save" as the page editor simply redirects to the login page again.

One could wish for a better auth-handling when editing in the admin interface: notify the use about the issue, let them copy the text and then redirect to the login (through a click on a new "LOGIN AGAIN" button).
- Pinging the webserver in the background would work too, making sure it never logs out the user when actually editing something.

Originally posted by @JornWildt in #423 (comment)