Add function in directory-indexer.py to add existing build

[justaddcoffee]

Describe the desired behavior

Right now in Jenkins we manually add a (blank) directory for each existing build in our projectdir: https://github.com/Knowledge-Graph-Hub/kg-covid-19/blob/4a1fda723602b90f2ef22da43452f15a82e883ed/Jenkinsfile#L175 on the command-line.

A better solution would be to add this functionality to directory-indexer.py on our fork of go-site

Additional context

Basically we'd just add flags for the bucket and project directory, and some ignore args to ignore directories like raw and current. The existing builds would be added to the HTML as href links.