Filter default sites for items user setting

[jimsafley]

This is an attempt to resolve a conflict between the "assign_new_items" site setting and the "can-assign-items" user permission. Put simply, a site can be set to automatically assign itself to new items, but the user can lack the permission to assign items to the site. This conflict could lead to a broken "Site" tab on the item add page (see #2166).

This PR fixes the conflict by a) limiting the sites on the site selector sidebar to those that the current user has "can-assign-items" permission; and b) adding the sites that are "assign_new_items" but restricted by "can-assign-items" to the "Site" tab, giving users the opportunity to assign these restricted sites, but only on the item add page. A few other changes to the site selector element, the item adapter, and JS were necessary to accomplish this.