Show the dataset path in the sidebar when running sans-manifest
Show the dataset path in the sidebar when running sans-manifest

…instead of showing nothing where the normal dataset selection controls
would be.  Showing the dataset path helps the user know what local files
they're looking at.

This also handles the case when we have a loaded manifest, but are
showing a dataset path that's not in the manifest.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice