Zoom plugin: add ability to configure cohosts

[jouvin]

It'd be great to configure cohosts in the Zoom room configuration dialog. It would be particularly useful when you create a room on behalf of somebody else and also want to cohosts other than yourself, as you cannot do it through the Zoom interface after transferring the ownership to somebody else. BTW it would be great to have the option "add yourself as a cohost" in the same dialog rather than hidden in the menu attached to join button.