[APPS-1729] Remove regex in favour of subdomain API

:koala: :+1: 

Remove regex in favour of subdomain API, demonstrating best practices in our demo apps.

This uses a hard-coded `.zendesk.com` subdomain which will be incorrect in staging and acceptance environments, but should be good enough for a public demo app as it was completely broken in those environments anyway.

/cc @zendesk/quokka

### References
 - Jira link: https://zendesk.atlassian.net/browse/APPS-1729

### Risks
 - None