Bug 6419: Add StaffListsHome block to additional contents

Bug 6419: Add StaffListsHome block to additional contents

This patch adds a new `StaffListsHome` block to the HTML
Customisations options. This allows for the addition of a block of
content in a page-section at the bottom of the lists home
page.

NOTE: This one may highlight an issue with Panels directly followed by
page-sections.

Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>