Add "Send invitations" feature to Waiting List tab

[ravicious]

1. To the left of each submission on the Waiting List, there should be a checkbox.
2. Once someone selects at least one checkbox, we enable the "Send emails" button at the top of the page.
   1. It can be at the same spot as it used to be on the Results tab.
   2. When no checkbox is selected, the button should be disabled and the title of it should say "Please select at least one submission".
3. After clicking on "Send emails", the admin should be taken to another page where they can provide the number of hours people have to confirm the invitation.
4. After that, let's redirect them to Invited with a flash message saying that the selected people will be invited.
5. In contrast to sending invitations from the Invited tab, we don't need to account for bad news emails here.

The process behind sending selected submissions should be super tight so that it won't send emails to people who weren't selected by mistake. I was thinking about an approach where clicking on "Send emails" sends the selected IDs to the server, then on the next page with the input for hours we include those IDs as a hidden field. And when the admin finally submits the final form with the hours, we check if those IDs are actually on the waiting list. If not, then let's display an appropriate error message.

We should probably finish #153 first.