Remove the need for the PUBLIC_ENDPOINT environment variable when running API and UI on the same host

[viktorstrate]

The two use cases for the environment variable PHOTOVIEW_PUBLIC_ENDPOINT is:

• To provide Cross-Origin Resource Sharing (CORS)
• To add full URL paths for images.

But if Photoview is configured to use the same host for both the API and the UI, both of these are not needed. CORS is not needed as it's not Cross-Origin and full URL paths could just be the full path without the domain.