[APPS-1567] As an App developer, I can define an author url in my manifest

:koala: 

This PR exposes the author url from the manifest.

/cc @zendesk/quokka 

### REFERENCES
- [JIRA](https://zendesk.atlassian.net/browse/APPS-1567)
- [ZAT PR](https://github.com/zendesk/zendesk_apps_tools/pull/51)

### RISKS
- None