Add exercise

https://www.pivotaltracker.com/story/show/64099624
https://www.pivotaltracker.com/story/show/57869968

https://workoutwars-review.herokuapp.com/

Adds protection so that you can't click add exercise a bunch and get multiple adds of the same exercise. Also sets server validation so only 1 exercise can be added. 

Makes it so users who create the competition are set as the creator and are the only ones who can modify the competitions