Add milestone reference to removed milestone note

* adds old milestone reference to system notes
* assures milestone_id is set when in resource milestone event when
  milestone is removed

Changelog: added

app/models/milestone_note.rb

@@ -17,6 +17,7 @@ def note_html
 
   def note_text(html: false)
     format = milestone&.group_milestone? ? :name : :iid
-    event.remove? ? 'removed milestone' : "changed milestone to #{milestone.to_reference(project, format: format)}"
+    reference = milestone&.to_reference(project, format: format)
+    event.remove? ? "removed milestone #{reference}" : "changed milestone to #{reference}"
   end
 end

app/services/issuable_base_service.rb

@@ -290,6 +290,8 @@ def handle_description_updated(issuable)
   end
 
   def update(issuable)
+    old_associations = associations_before_update(issuable)
+
     initialize_callbacks!(issuable)
 
     prepare_update_params(issuable)
@@ -297,7 +299,6 @@ def update(issuable)
     filter_params(issuable)
 
     change_additional_attributes(issuable)
-    old_associations = associations_before_update(issuable)
 
     assign_requested_labels(issuable)
     assign_requested_assignees(issuable)

ee/app/services/ee/issuable_base_service.rb

@@ -11,9 +11,7 @@ module IssuableBaseService
     def associations_before_update(issuable)
       associations = super
 
-      if issuable.escalation_policies_available? && issuable.escalation_status
-        associations[:escalation_policy] = issuable.escalation_status.policy
-      end
+      associations[:escalation_policy] = issuable.escalation_status&.policy if issuable.escalation_policies_available?
 
       associations
     end

spec/services/resource_events/synthetic_milestone_notes_builder_service_spec.rb

@@ -11,7 +11,8 @@
     let_it_be(:events) do
       [
         create(:resource_milestone_event, issue: issue, milestone: milestone, action: :add, created_at: '2020-01-01 04:00'),
-        create(:resource_milestone_event, issue: issue, milestone: milestone, action: :remove, created_at: '2020-01-02 08:00')
+        create(:resource_milestone_event, issue: issue, milestone: milestone, action: :remove, created_at: '2020-01-02 08:00'),
+        create(:resource_milestone_event, issue: issue, milestone: nil, action: :remove, created_at: '2020-01-02 08:00')
       ]
     end
 
@@ -22,7 +23,8 @@
       expect(notes.map(&:note)).to eq(
         [
           "changed milestone to %#{milestone.iid}",
-          'removed milestone'
+          "removed milestone %#{milestone.iid}",
+          "removed milestone "
         ])
     end