Small fixes and improvements

[ngc92]

Each commit here is a mostly independent fix or usability improvement. I didn't want to create a whole bunch of PRs for this, but if there is any non-trivial problem here, I'd just separate that commit out into its own PR.

Mostly small code cleanups, but also some small usability improvements:

1. leaderboard creation checks that deadline is not in the past
2. if a leaderboard allows exactly one GPU, the GPU selection popup is avoided (really nice for debugging)