Chart showing issues created per month

What does this MR do?

This MR adds a chart showing issues created per month for groups.

Issues Chart	Empty State (No Data)	Empty State (Filters)

closes #7478 (closed)

What are the relevant issue numbers?

#7478 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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? Yes, this is handled in https://gitlab.com/gitlab-org/gitlab-ee/issues/8037.

db/fixtures/development/09_issues.rb

@@ -8,7 +8,8 @@
         description: FFaker::Lorem.sentence,
         state: ['opened', 'closed'].sample,
         milestone: project.milestones.sample,
-        assignees: [project.team.users.sample]
+        assignee_ids: [project.team.users.sample&.id],
+        created_at: rand(5).months.ago
       }
 
       Issues::CreateService.new(project, project.team.users.sample, issue_params).execute