update migrate.sh to run tasks after reindexing (e.g. metadata extraction on files)

[zzacharo]

From @ppanero

Metadata extraction

When we create a record/draft with files, on file commit the extract metadata processor kicks in adding certain metadata to the file object inside the record. See https://github.com/inveniosoftware/invenio-records-resources/blob/master/invenio_records_resources/services/files/components/processor.py#L25.

Next actions

When migrating records/drafts we are not taking this step into account during reindexing. We could add then during transformation but I think is safer to trigger a task per record and drafts once reindexed. As it is configured by default there is only image size extraction https://github.com/inveniosoftware/invenio-records-resources/blob/master/invenio_records_resources/services/files/config.py#L58.