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.
Merge pull request #1260 from Ichimonji10/readable-entities

Add EntityReadMixin to some entities