Merge branch 'ref-ext-external-issues' into 'master'

Move reference extractor EE special case to separate method.

Fixes gitlab/gitlabhq#2370.

@dzaporozhets With this change, the part

```ruby
    %i(user label issue merge_request snippet commit commit_range).each do |type|
      define_method("#{type}s") do
        references[type]
      end
    end
```

is the same as in CE again, and the only added part is the issues method, which should not conflict with CE stuff.

See merge request !415

lib/gitlab/reference_extractor.rb

@@ -15,11 +15,15 @@ module Gitlab
 
     %i(user label issue merge_request snippet commit commit_range).each do |type|
       define_method("#{type}s") do
-        if type == :issue && project.jira_tracker?
-          pipeline_result(:external_issue)
-        else
-          references[type]
-        end
+        references[type]
+      end
+    end
+
+    def issues
+      if project.jira_tracker?
+        references[:external_issue]
+      else
+        references[:issue]
       end
     end