FacebookProvider: move to client-side flow
FacebookProvider: move to client-side flow

See:
https://developers.facebook.com/docs/authentication/#client-side-flow

Fixes: https://bugzilla.gnome.org/672060

ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,Distrotech/gnome-online-accounts,halfline/gnome-online-accounts