Clean up mark as read/unread star/unstar options

Mark read/unstar always operate on all messages in the selected
conversations.  Mark unread/star now work the same way everywhere: it'll
mark each selected conversation's latest message in the current folder,
falling back to the latest message period for folders like the search
folder where nothing really lives.

The individual mail operations available in the conversation viewer are
unaffected, as they always just operate on the one message.

Closes: bgo #714008
Rejigger indices to make slow queries fast
Merge branch 'master' into wip/725929-db-opt