Handle multiple karmas on bugs and testcases properly.

We have logic in the "traditional" karma system to detect duplicate karma from
single users and handle it.  I.e., if a user comes in and gives a +1 karma, but
then comes back in and gives a -1 karma, the first +1 should be ignored and
only the last -1 kept when calculating the 'total' karma for an update.
Similarly, if a user comes in and gives +1 karma, then comments again with
*another* +1 karma, that shouldn't count twice -- only once.

The changes in 6b12b16 should give us that same behavior but for bugs and
testcases.