webstaff: confirm dialog for void billings should avoid listing billing IDs

As a result of the patch for bug 1671603, the void billings actions in the web staff client now interpose a confirmation dialog. That confirmation dialog displays the total amount to be avoid as well as a list of billing IDs.

Such lists of billing IDs can quickly get very long and spill past the bounds of the modal's title bar. This is a bit untidy; options for improvement include:

- acting like the equivalent feature in XUL, where the confirmation dialog listed the billable xact IDs rather than the IDs of the individual billings
- not including a list of IDs at all, on the basis that the ones that the patron had selected are visible in the grid below the modal
- display, in the body of the modal, a list of the billable xacts that are to be voided, including id, date, and amount (and maybe type and title)

Note that a general UX idea I'm proposing here is that we should avoid displaying lists of bare IDs in confirmation dialogs in favor of listing more information about the records to be updated.

Evergreen 2.12