Harvest autoarchive

[abulte]

Fix datagouv/data.gouv.fr#157

This PR:

• adds an "archived" badge to the datasets list in admin
• adds an "archived" badge to the dataset view in admin
• adds an autoarchive attribute to HarvestSource (defaults to True)
• adds an autoarchive steps to the harvest process: archives datasets that are attached to the current harvester and not present on the remote platform
• fixes a bug on active field on the HarvestSource form
• exposes the dataset.archived attr in the API (read and write)
• support HARVEST_AUTOARCHIVE_GRACE_DAYS setting: do not archive unless the last harvest date has reached today - HARVEST_AUTOARCHIVE_GRACE_DAYS. This is to avoid archiving datasets that temporarily disappear from the remote platform (due to remote platform bug or maybe harvesting logic flaw)

TODO:

• tests
• expose archived datasets to the harvest report if possible
• expose HarvestSource.autoarchive on the harvester form in the admin
• test how many and which datasets would be impacted on production
• decide if we really want HarvestSource.autoarchive to be True by default

$ udata harvest run 5dfa334e1077e4b5aec33000
➢ Harvesting source "5dfa334e1077e4b5aec33000"
DEBUG: Initializing backend
DEBUG: Queued 10 items
DEBUG: Processing: wisconsin-polling-places
DEBUG: Processing: us-national-foreclosure-statistics-january-2012
DEBUG: Processing: gold-prices-london-1950-2008-monthly
DEBUG: Processing: afghanistan-election-districts
DEBUG: Processing: varicella-chickenpox-incidence-and-mortality-and-after-vaccine
DEBUG: Processing: us-tobacco-usage-statistics
DEBUG: Processing: state-workforce-generation-2011-2015
DEBUG: Processing: crime-data-ten-most-populous-cities-us
DEBUG: Processing: london-deprivation-index
DEBUG: Processing: florida-bike-lanes
DEBUG: Running autoarchive
DEBUG: Archiving dataset 5e0083cf1077e41d4a255e77: A archivér