[ENG-2482] add format_metadata_records management command

[aaxelb]

add a format_metadata_records management command meant to be used when we add a new metadata format -- queues non-urgent ingest tasks for all suids that will generate FormattedMetadataRecords in the given format(s)

side effects:

• update ingestion logic to kinda pretend that ingest jobs and suids are already one-to-one -- always use the most recent RawDatum for the job's suid
• add SourceUniqueIdentifier.ingest_job to get most recent ingest job for a suid (will someday soon be replaced with a proper OneToOneField)
• add SourceUniqueIdentifier.most_recent_raw_datum()
• add ingest task params:
  • pls_format_metadata: whether or not to follow the "new" FormattedMetadataRecord ingestion path (the "old" ShareObject path is controlled by the apply_changes param)
  • metadata_formats: which formats to create FMRs for (so running format_metadata_records won't do unnecessary, redundant work)

[coveralls]

Coverage decreased (-0.004%) to 79.639% when pulling 7c3dfd47fa6caaebe92a83f7caa4891d44341b1c on aaxelb:eng-2482--backfill-new-format into d424e2eec511387a481f412a73161922bd9b2686 on CenterForOpenScience:develop.