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


Merge issues came up on [old pull request](https://github.com/lighthouse-labs/laser_shark/pull/55). Created new branch