Discussions overflow x because of <code> and whitespace: pre

Thanks to the help of @rspeicher, we were able to figure out that the wild word wrapping in discussion sections as can be seen here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3111#note_4178408 is due to the <code> section of the comment and whitespace: pre. This style is needed but we need an alternative so that the whole comment section does not overflow but just the code.

@creamzy @skyruler @alfredo1 @iamphill