Allow to use issue/issues and merge_request/merge_requests for issuable_type in Insights configuration
As can be seen in gitlab-com/Product!5 (diffs), sometimes, people will use issuable_type: issues
instead of issuable_type: issue
and in that case the chart would render an error.
I think it would be nice to support the plural form since the chart itself returns data for many issuables...