Work request changes for workflows (before Workflow implementation) (Closes: #337)

Related to: #337 (closed)

• new system for blocked work requests: blocked status, unblock_strategy and dependencies fields,
  • and work-request-completed view changes to unblock reverse-dependencies as needed
• event_reactions field and its implementation in the work-request-completed view
• parent field (cf. !719 (merged))
• workflow_data field
• migrate existing notifications code to event_reactions [mostly ready but needs a couple more checks]