https://support.astron.nl/jira/browse/SDC-475

(and to read serializer, not yet to write serializer)

added /atdb/tasks/?predecessor__status=defining filter options to REST API

add 'resume' filter (hold/start)
removed 'defining' filters
added nicer 'Filter' button

- adding job table
see https://support.astron.nl/jira/browse/SDC-142

- priority default to 0
- sizes to BigPositiveInteger

adding status buttons to GUI

skeleton for 'get_size' endpoint

- skip -> resume
- added task.metrics

This reverts merge request !1