Minor fixes

refs #5425

Misc minor fixes: update_package_status idempotent (removes LOCKSS URLs
before adding new ones), download path returns correct URL for tar changes,
others.
Update pointer file - delete local copy

refs #5425

Delete LOCKSS chunks once AIP is safely stored, and remove their local URLs
from the pointer file.  If configured, delete local copy of AIP and update
pointer file: add deletion event, remove local URLs, etc.  Ensure that
multiple delete events not added if update status called repeatedly.
Download URL is absolute

refs #5425

Require the externally available URL to be configured for LOM Spaces, as
programatically determining the IP is unreliable.  Download URLs provided
to Lockss-o-matic prepend this to the relative path.

Service Document IRI converted to URLField to ensure it is a valid URL.
Collection IRI left as CharField, as it is read from the SD, and any
validation error would probably get buried.