Introduce the Insights controllers

What does this MR do?

• This first needs https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9912.

This implements a first iteration of the Insights controllers:

• Insights configuration is fetched from the current project's .gitlab/insights.yml or the from the group's Insights project's .gitlab/insights.yml file
• For now, the Insights page displays the Insight config and the chart data, just for the sake of testing, until we plug the Frontend on it
  • Group Insights:
  • Project Insights:

What are the relevant issue numbers?

Closes #9867 (closed), https://gitlab.com/gitlab-org/gitlab-ee/issues/10421.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• 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

app/views/layouts/nav/sidebar/_group.html.haml

@@ -44,6 +44,8 @@
                   %span
                     Contribution Analytics
 
+            = render_if_exists 'layouts/nav/group_insights_link'
+
       = render_if_exists "layouts/nav/ee/epic_link", group: @group
 
       - if group_sidebar_link?(:issues)