Issues with adding more organizers

[Didayolo]

Issues with adding more organizers.

1. In competition.yaml

• No "organizers" field in competition.yaml

It seems that we don't have a admins or organizers field in the file competition.yaml in order to easily add other users as organizers of the challenge.

• Update unpacker v1 (admins --> collaborators): https://github.com/codalab/codabench/blob/develop/src/apps/competitions/unpackers/v1.py
• Update unpacker v2: https://github.com/codalab/codabench/blob/develop/src/apps/competitions/unpackers/v2.py
• Update the wiki: https://github.com/codalab/codabench/wiki/Yaml-Structure

2. From competition creation UI

• Organizers are not added when creating a competition using the UI

When you create a competition using the UI, you can add admins from the interface but when you save the competition, the admins are not there. Updating the competition with the admins works.