Support https requests

The oae-rest module should not assume http as the default protocol.
The following should happen:

- [ ] The RestContext constructor should have an option that allows you to specify if `https` should be used
- [x] `RestAPI.Admin.loginOnTenant` should use the same http protocol setting as the global admin rest context
- [x] The preview processor should be protocol agnostic
- [ ] Optionally rename `userId` to `username`