added Multiple changes which are described in description.

1. added register and login functionality so users can sign up and login, users registration details except the password is sent to firstore and a username is generated out of their first and last name.
2. added a logout button to the header which allows a user to log out.
3. added an authHelper.js file in order to keep the register, login and logout functions together.
4. made functionality to upload reviews to the different holiday destinations, which then updates the review section straight away displaying their username, date they made the review, start rating(out of 5) and the actual review, done by storing these fields to a firestore document and then retrieving them. (made sure only signed in users can do this).
5. changed the review count from a static number(1) to a counter which counts items in the fetchReview array and displays it above the reviews
6. changed the average rating for that holiday from a hardcoded 4.6 to the actual average of the star ratings in the reviews.