Apply triage shortcuts to multiple selected emails at once

Requested via support by a power user working through large eDiscovery mailboxes, as follow-up polish to the now-shipped z/x/c triage shortcuts.

When several emails are selected via their checkboxes, the z / x / c shortcuts currently only act on the single highlighted row.

The shortcuts should apply to the whole multi-selection — so you can tick a batch of emails and approve / flag / trash all of them with one keypress.