Move group policies for group wikis into EE

What does this MR do?

This wasn't decided yet when we started working on it, but the feature also hasn't been released yet so there shouldn't be any implications for this move.

This also refactors wiki_policies_shared_examples.rb to verify the permissions in all situations, and fixes a bug where :download_wiki_code should be available on public or internal groups, even if the user is not logged in or a member.

Ref: #216242 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team