User help center API to fetch article contents to avoid CORS issues

:koala: :+1: 

If an agent has switched off SSL for help center and has a custom mapped domain the request will fail for articles that require authentication. Use the Help Center API to fetch the article instead.

/cc @zendesk/quokka 

### References

* [APPS-1520](https://zendesk.atlassian.net/browse/APPS-1520)
* [APPS-1532](https://zendesk.atlassian.net/browse/APPS-1532)

### Risks 

None.