never ending phase has null status

Hi,

Phases without without end date have "status": null (example). Apparently this sometimes breaks submission, with the submission form not being available to users. I have not experienced that (I'm an admin on tested competitions) but it was reported by two participants here. This was "solved" on this particular challenge by setting an end date. Participant were not able to submit before, except on some unknown occasion (ending date was added on June 8, some submissions could be made on June 7, but reports were made after and before those submissions…).

The issue might be related to this line: https://github.com/codalab/codabench/blob/b43105a73a789a3223cf14b7c4c1640fddda1867/src/static/riot/competitions/detail/submission_upload.tag#L3 but a better fix would be to have "status": "Current" for phases without an end date. This could also be the result of an issue with the phase indexing.

Thank you @BrunetNathan for the initial report