Improvements to the messages thread interface.

* Introduce noscript support for bulk actions.
* Revise structure and markup to better match the Notifications table.
* Add "Read/Unread" and "Delete" links for each message thread.
* Remove unnecessary "Select Read/Unread" dropdown.
* Introduce a number of new template functions in support of the new interface.

Props lakrisgubben.
See #6005.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9161 cdf35c40-ae34-48e0-9cc9-0c9da1808c22