Add weights to graph data

Use union of events to reduce queries

Extract scopes and use Gitlab::SQL::Union

Refactor and extract logic out of resource events

Apply suggestion to app/models/concerns/issue_resource_event.rb
Apply suggestion to ee/app/services/burnup_chart_service.rb
Apply suggestion to ee/app/services/burnup_chart_service.rb
Change scope name

Make attribute readers private

Use field names instead of index access