Change Provider Selection score ranking

Until this point Provider Selection score ranking was based on Provider Account priority. That's removed, so we change this to a more sane implementation:
- lower score means lower rank
- higher score means higher rank
- nil score means 0 rank
Fix tests after changing Provider Selection score ranking
PoolProviderAccountOption model
PoolProviderAccount controller and views
PoolProviderAccountOption tests
Weighting strategy
Weighting strategy tests
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