schema: Revert commit 74b8078a2de9993055b7c3b5745f7faaf38f817e as
 puavoSchoolHomepageURL was used instead.
schema: Add new attribute puavoFeed to store information about
schema: Add new attribute puavoExternalFeed to store information
about external feed sources that are associated with organisation,
school, group, user or device. This can be used to configure
information that should be displayed to a given user or device.