Customizing navbar

Would be neat if we could override the navigation bar used on the status page via `config.json` and maybe some custom HTML file we include. This would be handy when trying to make it easy to get back to other pages accessible from the home page. Also it would help make the status page feel more similar to the home page.

Start by tracing how the status page navigation bar is generated and how config.json is loaded; the issue also proposes a custom HTML file. Clarify the supported override format, then verify that a generated status page can use the custom navigation while preserving existing behavior.

Written by the indexing model from the issue text.