make enter collapse current message in thread-view-mode

make enter collapse current message in thread-view-mode

Pressing enter while over a text region in thread-view-mode will now
collapse the current message and move the cursor to the next open
message. (Behavior on quoted regions, sigs, attachments, etc. is
unchanged.)

This means that you can scan through a thread pretty rapidly by pressing
enter if you want to collapse the current message, or 'n' if you want
to keep it open.