Recognize stubbing of described_class in RSpec/SubjectStub

[lovro-bikic]

This PR makes stubbing of described_class register an offense for RSpec/SubjectStub cop. I think this fits under this cop since its purpose is to prevent stubbing the system under test, and described_class is just that.

To give a production code example offense: I recently noticed a spec for a module that had a allow(described_class).to receive(:some_module_method).

---

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.