Merging events and resource milestone events

Use sorting by id instead of created_at

Remove obsolete methods

Remove events merge

Using distinct issue_ids

Also modify issue retrieval statement to
improve performance

Change changelog title to reflect new scope

Fix visible issues

Store previously assigned milestones

Apply suggestion to ee/app/models/burnup.rb
Apply suggestion to ee/app/models/burnup.rb
Apply suggestion to ee/app/models/burnup.rb
Apply suggestion to ee/app/models/burnup.rb

Make Burnup a BurnupChartService