Enable holoviews based plots in figures and embeds

[stevejpurves]

PR #544 fixed holoviews outputs within notebook based pages by ensuring the holoviews cell outputs were properly reconstructed from minified mime-bundles.

Holoviews works on notebook based pages because an early cell adds the required scripts to the page in order for the cell outputs to load properly.

When an output is embedded as a figure on another page, those scripts are not loaded. We need a mechanism by which the cells with vnd.holoviews.load.json mime-bundles can be loaded on demand ahead of the vnd.holoviews.exec.json mimebundle being rendered.