ringaccount: improve banned contact handling

* addContact/removeContact have no effect if the contact is already
  added/removed
* removeContact: works even if contact didn't exist
* removeContact: add parameter to ban or just remove the contact
* sync: properly sync banned status

Change-Id: Ic76c02e7380b7de32803c02a28230341ae0ee802
Reviewed-by: Aline Bonnet <aline.bonnet@savoirfairelinux.com>