Support deleting several records at once

Per https://github.com/Kinto/kinto/issues/1287, right now, to delete records, you click on the little trash can logo, and then answer "Are you sure?" It would be nice to check a bunch of records and delete them all at once, answering the "Are you sure" question only once.

Start by reviewing the linked Kinto issue 1287 and the existing trash-can deletion flow in the admin UI. Identify how records are selected and how the confirmation is shown. Done means users can select several records, delete them together, and answer the confirmation only once.

Written by the indexing model from the issue text.