LP#1938835: customizable staff portal - schema, IDL, and seed data

LP#1938835: customizable staff portal - schema, IDL, and seed data

This patch adds two new database tables and corresponding IDL classes:

* config.ui_staff_portal_page_entry_type (types of portal widgets)
* config.ui_staff_portal_page_entry (entries/widgets for the portal)

It also adds a new permission, ADMIN_STAFF_PORTAL_PAGE, for managing
the new entries, seed data for the stock portal, and a grid setting
for the portal admin interface.

Sponsored-by: Pioneer Library System
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>