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.

ee/app/views/groups/issues_analytics/show.html.haml

 = render 'shared/issuable/search_bar', type: :issues_analytics
-
-%h1
-  = @chart_data
+- if @chart_data
+  #js-issue-analytics-loading.issue-analytics-loading.text-center
+    = sprite_icon('spinner', size: 72, css_class: 'fa-spin')
+  %h1
+    = _('Issues created per month')
+  %canvas#js-issue-analytics-chart{ data: { values: @chart_data.to_json }}