Handle overlapping appointments

[alangshall]

Currently the app allows for overlapping appointments. While there may be cases that a therapist intentionally wants to book overlapping appointments, there should be verification that this is intended.

Steps to reproduce:

1. Log into the app.
2. Create a new appointment lasting thirty minutes.
3. Create another appointment with a start time 10 minutes after the first one.
   Notice -> You are not prohibited from doing so, and there is no warning to the user.