feature: Set the callback URL at app creation time

[Roming22]

The homepage and webhook URLs can be set at creation time, but the callback URL cannot.
This issue is about adding the option of setting that field during the app creation process.

[gmlewis]

Would you like to submit a PR for this, @Roming22?
If not, could you please add a bit more detail, providing links into the code for what you are talking about so that one of our other contributors could take on this issue?

[Roming22]

@gmlewis The PR is live and ready to be reviewed. It is my understanding that the section I am modifying is currently not covered by unit tests. However I did test the change successfully in my application.