Change prompt-retirement to require more negative than votes to retire

Or, as expressed here, more (or equal) positive votes than negative in
order to preserve a prompt.

However the logic is worded, the point is that the old rule "Retire
any prompt with no votes" had a serious flaw: When someone submitted a
new category _just_ before a round started, nobody would get a chance
to vote for it, and then it would be dropped before anyone would see
it on the next round.

Now, instead, just like most everything else in empathy, the retiring
of an undesired category is now determined by a vote.