LinkedAccount dialog: fixed explanatory string

Fixed: https://bugzilla.gnome.org/show_bug.cgi?id=727805
LinkedAccount: fixed UI issues

Persona information for each linked account need to be improved but, as
long as UI issues are concerned, this is fixed.
Fixed: https://bugzilla.gnome.org/show_bug.cgi?id=727806
AccountList: use add_separtor utility func