[bp/r52823][FIX] forums: Cope with text parts nested in other parts in the mail which seems to happen when files are attached sometimes.
Also avoid parsing strings like "-----Original Message-----" as strikethrough, and replace "smart quotes" with normal ones.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@52825 b456876b-0849-0410-b77d-98878d47e9d5