Deal with zombie project nodes (nonexistent, renamed, moved, etc.)

[bugfolder]

This is a follow-on to to #1040. We've noted that when collecting GH statistics in borg_project_metrics module, GitHub returns HTTP error codes for a handful of modules, some of which no longer exist, or have been renamed, or don't have releases.

This issue is to decide what we want to do about them (leave and ignore, delete them from b.org, or something else?)