Weekend Time/57 -- Don't display time on weekends

[Trello Card](https://trello.com/c/HEihTyLf/57-weekend-schedule-don-t-display-times)

Students can choose to spread out the work over Sat/Sun as they please.

start_time will still be specified to facilitate ordering. The estimated duration should be displayed in it's place instead
WAY too much code for coming up with time. Also should be a view helper directly (defined in `app/helpers`, instead of a controller method that is turned into a helper using `helper_method`.

Also, if you check the db schema (`schema.rb` file) you will find that duration (in minutes) is actually a field in the activities table, whereas `end_time` is calculated by the model.