Runtime: Dashboard components with generic renderers and optional inline definition

[begelundmuller]

Changes:

• Changed the components: property to items: in the dashboard YAML
  • The idea is that item = component + grid info
• Renamed the Chart resource to Component
• Supports inline component definition from a dashboard:
  • if items[*].component is a string, it is treated as a reference to an externally defined component
  • if items[*].component is a map, it is treated as an inline component definition
• Added generic renderer and renderer_properties fields to Component
• Defines three renderers with the following formats:
  • "renderer": "vega_lite", "rendererProperties": {"spec": "..."}
  • "renderer": "markdown", "rendererProperties": {"contents": "..."}
  • "renderer": "image", "rendererProperties": {"url": "..."}
• Inline component definitions are emitted as separate Component resources called [dashboard name]--component[idx]
• Changes the /charts/{name}/data API to /components/{name}/data
• Changes the GenerateChartSpec API to GenerateRenderer. It now returns two values: renderer and renderer_properties.

See this design doc for the new component syntax.

This PR currently breaks the frontend since these are breaking changes.