#3469235: Make "Page data" tab in right sidebar work

The page data form is rendered in a similar way to the component props form: a backend route renders the entity form via the Semi-Coupled Theme Engine, the client app fetches it via an HTTP request, maps the JSX markup to React components via Hyperscriptify, then displays it under the Page data tab. See !339 (closed) to learn about the concepts and approach.

• Ensure that the entire markup is ready for Hyperscriptify, fix backend response
• Display the form under the Page data tab in the sidebar
• Hide submit buttons
• Fix layout problems
  • Area where the form is displayed needs to be scrollable — handled in !229 (merged)
  • Make sure no element has a fixed width
• Complete mapping for missing templates
  • Fix missing theme registry info
  • Vertical tabs → Accordion
  • Text format containers
  • Checkbox & radio
• Refactor DummyPropsEditForm and PageDataForm components
• Write simple end-to-end test for displaying the page data form

Outside of scope:

• Fully polished styling;
• Connecting to the Redux store;
• Sidebar UI/interaction changes:
  • Page data tab needs to be the first, default tab
  • The sidebar should always be shown, even when a component is not selected
  • The Settings tab (component props form) should have an empty state