Fix grades

* Calculate student percent now returns ```(percent * 100).round(1)```
* ```Course.letter_grade``` and ```Enrollment.letter_grade```(not sure this is used anywhere) now evaluates for whole numbers
* Changed views everywhere ```Course.letter_grade``` is used to look something like this:  ```<%= @percent = @student.calculate_percent(course) %>% <%= Course.letter_grade(@percent) %>```
Instead of: ```(<%= ((@percent = ((@student.calculate_percent(@course)))) * 100).round(1)%>% <%= (Course.letter_grade(@percent)) %>)```