[FIX] mail: mark document thread preview as read

Before this commit, it was not possible to mark a document
thread preview as read from the messaging menu.

This issue comes from the fact that document message previews
are built from messages in the Inbox. The computation of
the unread counter is correct for the preview, but not for
the associated document thread object which has a counter of 0.
Since it is only possible to mark as read when the counter is
strictly greater than 0, the 'mark as read' did not have any
impact.

This commit fixes this issue by ensuring that the unread counter
of a document thread matches the number of needaction messages it
contains.

Task-ID 1875668