Make it possible to run Flash in TBB
As one of the early steps for #7008 (moved), we need to fix all these bugs where people try to enable flash in TBB and can't. It seems that some of the problem is documentation (our instructions don't work), and some is that all the various things we do to disable plugins aren't actually as clean to disable as we hoped.
I'm using this as a catch-all ticket to round up all the tickets about this issue.