Each subtask row has a checkbox that marks the subtask card complete
(dueComplete, the card's own "Mark as complete"), and the Subtasks heading
shows "Subtasks (1/3)". A subtask is done when it is marked complete or
archived, which the minicard's subtask badge and "Hide completed subtasks"
now also count.
The setSubtaskDone method checks the edit right on the subtask itself, which
may be on another board; comment-only, read-only, worker and non-members are
refused, an assigned-only member may tick only a subtask assigned to them,
and an archived subtask is refused. The change is recorded in the subtask's
History like the card details toggle.
The new strings are in English in every locale, pending Transifex.
Thanks to zhoujunhaohao and xet7 !
Fixes#4693.
