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.

doc/user/group/issues_analytics/issues_created_per_month.md

@@ -3,7 +3,7 @@
 **Note:**
 Introduced in [GitLab Premium][ee] 11.5
 
-A bar chart of issues created per month for the last 12 months by default, the user can select n months back to be displayed. 
+A bar chart of issues created per month. It displays all issues created in the last 12 months by default, but it can be set to display x months.
 
 The top sidebar can be used for filtering the issues by any attribute like labels, assignee, milestone, author etc.