Prototype managing Content Templates within Canvas Workbench

Overview

Similar to #3576837: Prototype managing Regions within Canvas Workbench, we also want to prototype Content Templates within Canvas Workbench.

Proposed resolution

• Content Templates should appear below Pages in Workbench's left-hand navigation.
• For the initial prototype, Content Templates can be hard-coded as .json files, similar to Pages, and not yet sync'd with the Content Templates that are in Canvas.
• The Content Template .json files can use json-render's data-binding syntax to reference dynamic prop values.
• The prototype can depend on the jsonapi module.
• When viewing a Content Template in Workbench, there should be a dropdown to select which node to view. The list of nodes can be fetched from jsonapi (or a jsonapi wrapper, such as API Client).
• When a node is selected, the jsonapi response for that node (or some transformation of it such as what's returned by API Client) can be used as the $state passed to json-render.

User interface changes