Delete multiple tests at once

[cedpeters]

Currently, to delete multiple tests in the same repository, the user must open the detail view for each page and then authenticate.

Make it so users can tick checkboxes next to the tests they want to select, and then click "Delete Selected Tests" button at the bottom of the page. They will be authenticated with Github, and all of the tests they have permission to delete will be deleted.

Need to update the snackbar as well to tell the user which tests were and weren't deleted.