Message index CSS

Basically a fix for the problems discussed here:

https://github.com/elkarte/Elkarte/pull/1338
It's a full switch to display: table  -> table-row; -> table-cell, 

Tested with W7 and latest versions of Chrome, IE11, Firefox and Opera.

Sorry, can't test IE8+9 right now... From a quick research it should be supported with IE8+.