Terminology updates

[jcushman]

Some updates for clarity:

• "migration" becomes "changeset", so it doesn't imply that it can be automatically applied like a Django migration.
• "outputter" becomes "renderer", just less clunky
• "ministerial" becomes "clerical", more mainstream
• node "kind" becomes node "action" so it's less confusable with "item_type" -- example action values are "rename", "delete", "move" etc.

Add an ADR for terminology.