When we originally started working on this, it wasn't clear yet if it
would be a paid feature. Shortly afterwards we decided to put it into
EE Starter and moved the code into `ee/`, but forgot to add the license
checks.

- Rename feature flag to `group_wikis_feature_flag` to align with the
  feature name in the `License `model, which is plural as well.
  We have to use a different name for the FF because licensed features
  need to default to on.

- `EE::Group#wiki_access_level` is removed since this was only added
  to support the shared examples for wiki policies, which also expect
  the `private` access level to be supported. In practice we don't
  support this yet for group wikis, so now we just skip these specs
  instead.

- The license and feature flag checks are centralized in the group
  policy, so we only test these special cases there, and otherwise
  just call our new `stub_group_wikis` helper to test the general
  case of the feature not being available.