Rewrite score computing

Created by: njoyard

• Remove scores from DB
• Create a ScoredVotes view (vote, [representative, proposal, date], score)
• Rewrite the DossierScores view that aggregates entries from the previous one over dossiers
• Create a Scores view that aggregates entries from the previous one over representatives
• Remove score update facilities everywhere
• Update the API accordingly

Note: include hooks to set up different score aggregation rules (see #63)