Implement work request changes for workflows

Part of #276 (closed).

https://freexian-team.pages.debian.net/debusine/explanation/concepts.html#work-requests, https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflows.html#workrequest, and https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflows.html#expected-changes between them specify some changes to work requests that need to be implemented. We'll need:

• 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
• workflow_instance field (once #294 (closed) has landed, or as part of that issue)
• workflow_data field
• event_reactions field and its implementation in the work-request-completed view
• migrate existing notifications code to event_reactions