[BUG][APPS-1554] Support acceptance with HC + Custom Domain

:koala: :bug: :+1: 

When running the app on non zendesk.com domains, with HC and custom domain enabled, the regexp wouldn't match. See #33 .

/cc @zendesk/quokka 

### References

- [jira](https://zendesk.atlassian.net/browse/APPS-1554)

### Risks

- Missed other domains