Add EntityReadMixin to some entities

When asked for information about an entity, the API often returns data that is
inconsistently named, inconsistently structured, or incomplete. Thus, the
EntityReadMixin can only be added to a few entities. Do so, and add a test
verifying that the mixin works.
This change is not directly related to the PR. It just happens that this class name and its docstring are poor, and it's been bothering me for some time.