Define the authentication flow

[rbarrois]

Start with a design doc.

Needs to provide the following features:

• When I enter the new site while already connected on the old one, I don't see any password prompt
• When I log out of one of the sites, I am automatically logged out of the other one
• When I enter a sensitive area, I get a password prompt

[fishilico]

If authgroupex is modified so that:

• when doing a request with authreq=password in the query string, a password prompt always appears,
• when doing a request with authreq=weak or weak=password, the response contains a new field auth containing either weak or password according to the authentication process which took place

... then I believe authgroupex can be used to implement the new authentication process.

For the logout part, we can use the redirect parameter of the /exit page (https://github.com/Polytechnique-org/platal/blob/xorg/1.1.15/modules/platal.php#L588) to kill the main session and return to the new site.