The licenses table, along with the package_metadata table had duplicate
values. This could happen as the unique constraints on those tables didn't
properly account for the nullable fields.
The duplicates in those tables also affected the license_sets, packages,
package_derivations tables in a similar way. Finally, the
guix_revision_package_derivations table was also affected.
This commit adds a migration to fix the data, as well as the constraints. THe
code to populate the licenses and package_metadata tables is also updated.
And display this on the package page.
This uses a couple of new tables, and an additional field in the
package_metadata table.
Currently, the order of the licenses in the package definition isn't stored,
as I'm not sure the order in the list is significant.
