Bug 1067550: Add support for new get-involved page.

Not added to docs because custom endpoint for only one site.

@Osmose r?
I guess I just feel weird using the meant-for-a-human value as the flag for what specific error happened, and supposedly if the success case data is shared among all tests and you make just one modification here, you can be reasonably sure the failure is because of that modification. 

Either way compiling these tests into a helper method is still something you should do.