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.
Update LOM to stop harvesting before deleting files

refs #5425

Require LOM to be updated to stop harvesting before deleting local
copies of files.  If updating the package status fails, return an error
string to display.  Refactor code relating to delete files into its own
function.