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

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

@Osmose r?
I'm not a fan of splitting up the tests this way, because you're testing the success case of the function a bajillion times, and as soon as the success case changes all these tests have to change too.

Instead, have a test for each one of these cases contained in this method, and factor out `base_data` to be valid from the start and created in `setUp`.