Use more specific selectors in bp-legacy table CSS

The previous table styles were too general, which caused styles to leak
through inappropriately to non-BP tables. This was a particular problem for
the table-based syntax of rich text editors embedded via wp_editor().

Fixes #5167

Props netweblogic, hnla

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7595 cdf35c40-ae34-48e0-9cc9-0c9da1808c22