[#66908198]add enrollment_rate LocalStat calculator
[#66908198]make enrollment_rate a MonthlyStat (calculated locally by LocalStat)
[#66908198]add enrollment_rate to ranking
[#66908198]fix specs
[#66908198]add dropout_rate LocalStat calculator
[#66908198]fix prev month ref_date
[#66908198]dropout_rate is now a MonthlyStat
[#66908198]add dropout_rate to ranking
[#66908198] add rake tasks to calculate new rates

- fixed specs
[#66908198]add rates as default ranking columns
[#66908198]male students rate calculation
[#66908198]add male_students_rate to ranking and as monthly stat
[#66908198]rake to calculate male rate
Merge 66908198-rates-in-ranking to master

[Completes #66908198]