Add "Send invitations" & "Sending invitations" screens to Invited

[ravicious]

1. Remove the "Send emails" button from the Results tab.
2. When no invitation is sent yet, let's display a "Send invitation" form on the Invited tab.
   1. It should let the admin choose how many days those participants have to confirm the invitation through an input field (default to 7).
   2. We can check if any invitations were sent by checking if there are any submissions with non-null invitation_token_created_at.
   3. After sending the invitations, the "Send invitations" screen will be no longer seen. However, since we send emails asynchronously, there's the problem with what to show right after clicking "Show invitations"…
3. …in that case, let's display a "Sending invitations" screen on the Invited tab.
   1. It should refresh itself every 5 seconds.
   2. This screen should be displayed only if there are no submissions with invitation_token_created_at, but there are some delayed jobs in the queue.
4. Once we actually send at least one invitation, the Invited tab becomes a regular list, as described in Add "Waiting List" tab & rename "Participants" to "Invited" #151.

Blocked by #151.