Enable force-regeneration of static HTML content

[cdata]

As of #51 we will have landed a basic facility for generating static HTML from the full history of sphere content. In the basic case, we avoid re-generating HTML for any content revisions that we have already generated HTML for. However, there will be cases where the user may wish for HTML to be regenerated from scratch (e.g., we or they have changed the styles or layout in some way and the user wants to update older revisions to match newer ones). We should provide a mechanism for doing this easily.