Mark messages as read when moving to spam

Problem
Most of the time I move messages to spam without opening them. The message list view is enough. So I mark messages and choose Spam from the menu when messages are still marked as unread. Later, when I open the spam folder to see if there is any new ham, those messages are still marked as unread and unnecessarily make searching for spam more timeconsuming (I've got one account with a poor filter and a lot of false negatives).

Solution
When users choose Spam action, mark messages as read (if users configure so) when (before or after, whatever is more convenient to code for IMAP) moving to spam folder.

Start at the Spam action in the message list and trace the IMAP move-to-spam flow, including how the user's read-state preference is checked. Done means configured users have messages marked read when they move them to spam, while the existing behavior remains for users who do not enable it.