Move mark as read and archive to thread mode

This allows me to do searches to refine the crap in my inbox, and
`<shift>-a` them right away, instead of `| => is:unread label:<blah> =>
<shift>-T =<shift>N => | -label:<blah> | <shift>-T =a`

All I did was copy over the code to Thread Index Mode and test it. Would
love to write some tests for this if required, but I couldn't find any on particular for this (with a quick grep on read_and_archive in test/)

Cheers guys, love sup :-)