user-accounts: Reorganize the user-options container

This commit merges the hbox2, main-user-vbox, and grid1 into the
"user-options" container.

It also replaces deprecated widgets, such as GtkVBox and GtkHBox.

These changes are according to the new User Accounts panel mockups
at https://wiki.gnome.org/Design/SystemSettings/UserAccounts

https://bugzilla.gnome.org/show_bug.cgi?id=767065