Merge branch 'hotfix-epics-promotion' into 'master'

return true if milestone is not available for issuable

See merge request gitlab-org/gitlab-ee!9855

app/models/concerns/issuable.rb

@@ -121,7 +121,7 @@ module Issuable
     end
 
     def milestone_available?
-      return if is_a?(Epic)
+      return true if is_a?(Epic)
 
       project_id == milestone&.project_id || project.ancestors_upto.compact.include?(milestone&.group)
     end