Keyboard shortcut to select / check items in the email list (e.g. a)

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.

To build up a multi-selection one-handed, it would help to have a key that toggles the checkbox on the focused email — the customer suggested a, in keeping with the one-handed z / x / c scheme. Combined with the next/previous keys, you could move down the list and tick items without touching the mouse.