The way that the dashboard currently does batch operations is to iterate

through the entire list of checked items and do an API call for each

one. This works OK but is quite inefficient, and our new batch view

allows doing it all in a single HTTP request.
This change moves the close operations (submitted and discarded) over to

that new endpoint. While I was doing this, I updated it a bit to clean

up the code (preferring promises and async/await over callbacks, and

using the _ template string formatter). I also changed the finish

button on the dialog at the end of the process from "Thanks!" to "Close"

because that's always driven me batty.

Did a bunch of tests with various dashboard states and saw that

  everything was closed appropriately.
Set up fake cases where we'd get a subset of closures and a server

  error, and saw the expected error messages appear.
Ran unit tests.
Ran js-tests.