Optimize NumericQuestion#report_data.

- Don't do grouping in memory. Was very expensive…hitting the DB too many times.
- Now uses one query.
- Also use `to_f` instead of `to_i` for the contents. Decimal numbers were getting rounded down.
- Need to use `unscoped` because we don't want to order the answers by the question's order number (default scope in answer.rb)
Go through all the questions when creating <div>s for each question in the report page.

Survey#questions_with_report_data is too expensive.