Bug 6419: Add StaffSerialsHome block to additional contents

Bug 6419: Add StaffSerialsHome block to additional contents

This patch adds a new `StaffSerialsHome` 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 point of sale page.

NOTE: This patch also removes the now defunct database update introduced
in the first patch of the series.

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>