redo prebookCheck validation: UserServiceSelected

[rconjoe]

There is a check before you book anything that makes sure you don't already have anything booked during a given time that you're tryign to purchase for

it did weird dayjs comparasions that can just be integer comparasions of unix times now. This is holding up #72 because the validation is in the same function

[rconjoe]

I might just offload this onto the api and do some checks inside of the checkout call, and return some custom error code if you have overlaps

[rconjoe]

Delayed

This is going on the back burner for now. The checks for not-buying-your-own-session work, but overlap booking yourself is still possible.