Seed an approval kind and an apply step for convention changes

Seed the approval kind or kinds for a convention change, decide where the pending row lives, and add the step that applies the change when the last decider approves.

Market Risk has a deciding part and the permission inbox::approvals:decide_market_risk. Only iam.role_grant, refdata.book_change and refdata.counterparty_change are seeded as kinds, so the queue of a Market Risk approver cannot list a convention request. A request holds a kind and a reason and no pending change. Nothing applies a change on the last approval. The open choices are one kind for all four convention writes or one kind for each, and whether the pending row is on the request or staged in the convention table.

• Decide a convention change
• Name a convention set
• Retire a convention
• Book types and the controls on each