Prevent user from adding already excluded dates

[karismatic-megafauna]

Currently, when making a exclusion date, you can add multiple of the same dates but only if it is the date you just added or the current date.

Do one of the two options:

• option 1: Put error in error banner if user selects date that is already an exclusion date

• option 2: Prevent user from ever being able to add an exclusion date by having the newExclusionDate increment by one once addExclusionDate is called.

Option 2 is easier and wouldn't rely on the separate issue detailed in #42 of adding an alert system.

Thanks ❤️

[karismatic-megafauna]

https://momentjs.com/guides/#/lib-concepts/date-time-math/ might be useful in this issue :)