[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/13.x@52823 b456876b-0849-0410-b77d-98878d47e9d5
[MRG] Automatic merge, branches/13.x 52818 to 52823

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@52824 b456876b-0849-0410-b77d-98878d47e9d5