The big kahuna: preview workflow, tabbed UI refactor, dynamically generated APIs.

This MR is the result of two different streams of development that were merged because one started to depend on the other. Here are main features.

• Introduces the previewable workflow state and updated publication workflow. The environment variable PUBLICATION_PREVIEW_ENABLED toggles the availability of preview, and it is enabled by default.
• Refactors the tabbed UI on the resource show page to more cleanly separate configuration -- see config/tabs.yaml. Introduces a new "Workflow" tab.
• Transitioned the statically generated APIs to dynamically generated API classes.