Introduce a rubocop check for license available usage

What does this MR do?

• Introducing Namespace#licensed_feature_available? for consistency with Project#licensed_feature_available?: I believe in the end, Namespace#licensed_feature_available? should replace Namespace#feature_available? since Namespace only checks for licensed features.
• Introducing a new RuboCop check that:
  • Make sure Project#licensed_feature_available? is called when the given feature isn't included in ProjectFeature::FEATURES
  • Make sure #feature_available? is called with two arguments (for now this will warn for Namespace#feature_available? calls, but should stop warn once all Namespace#feature_available? calls will be changed to Namespace#licensed_feature_available?)

Note that the Gitlab/FeatureAvailableUsage cop is disabled in the HAML-Lint configuration since it's currently impossible to exclude offenses per file in HAML-Lint for a RuboCop check.

Related to #282298.

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