ProposalResult.update_vote doesn't work on delete

[chrisjrn]

Steps to reproduce:

• Add review with +1 score
• Overwrite review with +0 score
• Delete review with +1 score
• Delete review with +0 score

Expected:

• 0 x +1 votes
• 0 x +0 votes

Actual:

• 1 x +0 vote
• -1 x +1 vote

[chrisjrn]

Fundamentally the update_vote method is a bit of a shambles, and appears to be fundamentally broken on the delete case. My guess is that making full_calculate a bit more database efficient would be a far far far far better solution here.