Add "Waiting List" tab & rename "Participants" to "Invited"

[ravicious]

We'd like to add another tab to the app. It will be called Waiting List and it'll be placed between Results and Participants.

1. Waiting List includes all rated submissions which haven't received an invitation.
2. Participants becomes Invited.
   1. It includes all rated submissions which received an invitation.
   2. Next to each submission we display its status (invited/confirmed/expired).
   3. The download to CSV feature should include a separate column with the status as well.
   4. Let's add the download button to the top of the page while we're at it.