Show popup when signing into an account that has an appointment

[rotembarsela]

Steps to reproduce:

1. Book an appointment
2. Log out
3. Try to book a different appointment (as logged out)
4. Sign In

Expected Result:
The first appointment is shown,
as well as a popup explaining that the second appointment will not be booked.

Actual Result:
The first appointment is shown.

Moved from: Trello