End of authentication in the back-office

[mlebretonFR]

Currently, in Umbraco 15+, when the authentication is lost in the back-office (after a certain time), nothing happens.

• It's a security issue. Someone can SEE what's on the screen (it may be critical data). Even if it seems to be a little issue with a little impact, we all know that when people are away for a coffee, they rarely log out their computer.
• It's also a user experience issue. If you have not seen that you lost the authentication (because you were not using the back-office for a time), and if you don't navigate but stay on the current UI, as soon as you restart your work, you receive a lot of error messages.

Expected behaviour (in my opinion):

• Authentication is lost ⇒ immediate page reload ⇒ not displaying an overlay :a reload (so the authentication screen appears), like this, even manipulating the DOM via the console can't access to some data.
• Also, the actual re-authenticate popup is a bad idea (for the same reason).

General security rules for applications:

1. No authentication = login screen + the minimal version of everything (particularly the scripts). The back-office should not be completely loaded. No risk of any leak of anything. We don't give any information or any technical information to any hacker.
2. When the authentication is lost ⇒ reload ⇒ back to 1. The back-office is back to a minimal version.
3. After an authentication, ⇒ reload (not just displaying something already rendered). This time, all the back-office may be loaded.