Make `ContentView` readable, trim down smoke test

Make `ContentView` inherit from `EntityReadMixin`. use this mixin to trim down
module `tests.foreman.smoke.test_api_smoke`: replace many usages of `client.get`
with `ContentView.read_json`.
I assume that the intent here is not to check that the status code is an HTTP 200, per se, but instead to simply see if searching produced some sort of error. If so, I think using `raise_for_status` is a more concise way of doing so. That said, I don't have a super strong opinion, so just say if you disagree.