Default Insights chart for issues created per month

- Current default charts for issues rely on labels to be complete
- This provides a nice out of the box chart for issues

ee/changelogs/unreleased/insights-issues-created-per-month-default.yml

+---
+title: Changes to default insights charts
+merge_request:
+author:
+type: changed

ee/fixtures/insights/default.yml

 issues:
   title: Issues Dashboard
   charts:
+    - title: Issues created per month
+      type: bar
+      query:
+        issuable_type: issue
+        issuable_state: all
+        group_by: month
     - title: Bugs created per month by Priority
       type: stacked-bar
       query: