Advanced search - query work item milestones by special filters

The following milestone timestamps were denormalized milestone.start_date and milestone.due_date and moved to ES as milestone_start_date and milestone_due_date fields to support these milestone's scopes:

• Milestone.started
• Milestone.not_started
• Milestone.upcoming_ids
• Milestone.not_upcoming

Now we should update existing by_milestone ES filter and let it support :milestone_started, :milestone_not_started, :milestone_upcoming, and :milestone_not_upcoming filtering options in ee/lib/search/elastic/filters.rb.