Improve the Insights feature availability check

What does this MR do?

This creates a new InsightsFeature module that provides a new #insights_available? method that encapsulate the feature flag check which is mandatory, and then if it's enabled, check the license.

What are the relevant issue numbers?

This builds upon https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9665.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer