Added calendar component to allow users to add exercise for previous days
Updated backend to take date from front end
Small layout changes
Note: The deadline for exercises to be submitted at the end of the challenge week will be midnight, then the week will reset. There is 1 deadline per week in the current state. These improvements can made in a future iteration
Note: The deadline for exercises to be submitted at the end of the challenge week will be midnight, then the week will reset. There is 1 deadline per week in the current state. These improvements can made in a future iteration