Import maintainers when there are no maintainers

[williln]

closes #616

The problem was that when import_library_versions runs and loads the maintainers, it only tries to load them if the LibraryVersion was created. Since older versions of WinAPI had maintainers, and my local db had maintainers for the current version, it looks like the LibraryVersion for WinAPI-1.82.0 was probably created, but for whatever reasons its maintainers didn't load. And on subsequent runs of this import, the maintainers wouldn't be attempted since the LibraryVersion record already existed.

I changed the logic to try and import maintainers if there aren't any, regardless of whether the LibraryVersion record is new.

Once this is merged, ./manage.py import_library_versions --min-release=1.82 will need to be run in deployed environments.

[frankwiles]

LGTM!