New contribution score metric

[h-lame]

Because we're going to anonymise proposals (see [#18]) adding a proposal shouldn't affect the contribution score for the champs & scamps page any more - otherwise it becomes trivial to work out which people have proposed something (if not what they've proposed).

By removing that metric we might want to review how the scores are calculated to make sure we're promoting the best other behaviour. Maybe there are other things we can give you a score for?

[h-lame]

Proposals you've made no longer affect the contribution score (see: 1661063). Will leave it up to @lazyatom or @tomstuart to decide if that's enough before closing.

[lazyatom]

Looks good to me.