Support "unlisted" mods

I'd like to add my auto-generated (by this) Test Mod as an example, but it shouldn't appear in the list of mods. It should be accessible by URL.

Unlisted mods would be good for examples like this... they could be added to the documentation and the user wouldn't have to leave the site (unless they choose to look at the repo).

Unlisted mods would also be a good feature for mod authors to preview everything before "publishing". It would allow testing end-to-end (which isn't really completely possible because the version.txt has to exist on the site) before initially releasing something. In that case, maybe it would be good to add unpublished new releases as well as unpublished new mods.