Adding A/B/C testing to hint generation.
Fixes for test cases.
Merge from finished.
Updated based on Ellen's comment.
Updated get_best_next_hint to get_third_party_hint.
Merge pull request #826 from code-dot-org/finished

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Merge branch 'finished' into delivered
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Merge branch 'delivered'
Merge pull request #829 from code-dot-org/finished

DTD
Merge pull request #830 from code-dot-org/finished

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Merge pull request #831 from code-dot-org/finished

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Merge pull request #832 from code-dot-org/finished

DTD
Drop hints table, add source column to level_source_hints.
Update Stanford hint importing to changed schema.
Merge pull request #835 from code-dot-org/finished

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Merge pull request #836 from code-dot-org/finished

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Changed hints editor to only show crowdsourced hints.
Factored out string constant LevelSourceHint::CROWDSOURCED.
Merge pull request #837 from code-dot-org/finished

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