Add `mu-` prefix to all discussion CSS classes

[felipecalvo]

I am not asking to change this here, but I'd really recommend reinforcing the policy of adding mu- prefix to new css classes as opposed to legacy css code which lacked of good semantics, was difficult to customize and allowed name-clashes.

Originally posted by @flbulgarelli in #1644 (comment)

All discussion-* classes lack this prefix.