Remove Provider Account priority attribute
Delete tests for Provider Account priority attribute
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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