removing endorsements

[Eric Dean Tribble <tribble>]

All this talk of Un/Redo has made me revisit removal of endorsements.
We have to be able to remove endorsements, for fine-grained undo and
larger mistakes (specific examples upon request).

The last semantics MarkM and I agreed on for this was that a user
could remove one of his endorsements (so it wouldn't be used for
filtering), but that readers could still see if there used to be an
endorsement.  I can't think of any particularly elegant way to
implement this, though.  MarkM?

dean