Prevent issuable which don't support epics from being linked to epics

For example, incidents should be linked to epics.

ee/app/services/epic_issues/create_service.rb

@@ -50,7 +50,9 @@ module EpicIssues
         return [] unless can?(current_user, :admin_epic, issuable.group)
 
         issues.select do |issue|
-          issuable_group_descendants.include?(issue.project.group) && !previous_related_issuables.include?(issue)
+          issue.supports_epic? &&
+            issuable_group_descendants.include?(issue.project.group) &&
+            !previous_related_issuables.include?(issue)
         end
       end
     end

ee/spec/services/epic_issues/create_service_spec.rb

@@ -232,6 +232,14 @@ RSpec.describe EpicIssues::CreateService do
             )
           end
         end
+
+        context "when assigning issuable which don't support epics" do
+          let_it_be(:incident) { create(:incident, project: project) }
+
+          subject { assign_issue([incident.to_reference(full: true)]) }
+
+          include_examples 'returns an error'
+        end
       end
 
       context 'when user does not have permissions to link the issue' do