[FIX] forums: Don't reset  Perms_Context if it wasn't set in the first place, and check pref properly

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/13.x@52731 b456876b-0849-0410-b77d-98878d47e9d5
[FIX] forums: Make the recently added "parse html in incoming forum emails" optional with a new pref, as it often cannot cope with complicated table based html emails.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/13.x@52733 b456876b-0849-0410-b77d-98878d47e9d5