consider only displaying "used" Releases, unless unused requested

[gbrownmozilla]

There are a lot of Releases in the production db now, and new ones being added all the time. Some show "Used by: n/a", which I think means they are completely unreferenced and not in use. We may be keeping these around for auditing purposes ("have we ever shipped ..."?) But for normal UI inquiries, these are probably useless clutter (more entries to scroll through, longer wait to populate the list than necessary). We could filter out unused releases from the normal Releases list, unless requested by some new UI element.

[gabrielBusta]

This could be really straight forward. I'll look into it