Change freedombox task to use metapackage

Thank you for merging the patch in #834239, however I'm sorry to say the patch had become out-of-date, and I was not paying attention to keep it updated. The "freedombox-setup" package is now obsolete, and its contents were moved into "freedombox" package. In addition, we have the "fbx-all" metapackage which defines the tasks for Debian Blends.

Also updated the task description based on the freedombox package description. These changes were reviewed by the team in our recent progress call.