Thread:Feedback/"Mark as read" buttons/Werdna

I'm thinking of three immediate improvements to the new messages page in response to this feedback:
 * 1) Add rules between the rows in the table the new messages are shown in, to allow easily distinguishing them.
 * 2) Make the "mark as read" button follow the scroll position.
 * 3) Instead of deleting the row when a thread is marked as read, replace it with "The thread 'Blah blah' was marked as read. Undo".

I'm not fully clear on adding "Mark as read/unread" elsewhere. One of the things about the new messages architecture right now is that the read/unread state is only maintained for watched threads. If we expanded this to all threads then it might get a bit out of hand, performance-wise.