ttrss: sso: Cookie remains after user logs out of Plinth

Created by: JosephKiranBabu

Tiny Tiny RSS currently uses FreedomBox Single Sign On.

When a user signs into tt-rss from Plinth and later logs out of Plinth, a cookie set by tt-rss remains (name: ttrss_sid_ssl) which allows the next Plinth user to be able to view the feeds of the logged out user.

Flow of events:

1. User1 logs into Plinth.
2. User1 visits tt-rss.
3. User1 presses back button in browser, goes to Plinth and logs out.

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1. User2 logs into Plinth on the same browser.
2. User2 accesses tt-rss.
3. User2 gets the feeds of User1 since tt-rss still has the old cookie.

Here, User1 cannot even logout from tt-rss, since they'll be logged in back again immediately after clicking logout. tt-rss logout redirects back to tt-rss, which then accepts the auth_pubtkt cookie and logs in the user into the application. Maybe this should be considered a bug too.