books: Disable online accounts for now

Until we can figure out which of the providers could be populating
our view with ePubs or comics, just disable online accounts.

https://bugzilla.gnome.org/show_bug.cgi?id=740974
documents: Hide eBooks from Documents view

gnome-documents showed eBooks and Comics in the documents view because
they matched the "Paginated Document" RDF type. Make sure to exclude
paginated documents from the documents view.

https://bugzilla.gnome.org/show_bug.cgi?id=741594
books: Don't show "add online sources" when empty

If there are no books on the system, don't show the "Add online
sources" link to gnome-control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=740974