Finishes the refactoring started in c1bbd8d. The goal of the refactoring is
to change the argument to get_realm from a Realm.domain to a
Realm.string_id. The steps were
* Add a new function, get_realm_by_string_id.
* Change all calls to get_realm to use get_realm_by_string_id instead.
* Remove get_realm.
* (This commit) Rename get_realm_by_string_id to get_realm.
Part of a larger migration to remove the Realm.domain field entirely.
Clean up the instances of self.assertIn("string", result.content.decode("utf-8")),
and replace them with self.assert_in_response("string").
Fixes: #2313
This makes us more consistent, since we have other wrappers
like client_patch, client_put, and client_delete.
Wrapping also will facilitate instrumentation of our posting code.