user-accounts: Rename "Automatic Login" to "Login Authentication"

This also inverts the boolean action because automatic login ON
means login authentication OFF and vice-versa.

According to the mockups at
https://gitlab.gnome.org/Teams/Design/settings-mockups/raw/master/users/users.png

Fixes #771
11 jobs for wip/feborges/user-accounts-ui-improvements in 2 minutes and 57 seconds (queued for 1 second)
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