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

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

@Osmose r?
This smells like an `assert_error_response` method that does all these assertions for you, with arguments for the specific messages and such. I'd argue testing the `desc` of the error isn't necessary, rather testing that an error happened and it has the right error code is better (both HTTP and special basket code for things like usage error).