Redirect issue if repo has configured external tracker. (#9247)

* Redirect issue if repo has configured external tracker.

* Handle error

* Add tests for redirect

* Fix test consistency

models/fixtures/issue.yml

@@ -97,3 +97,15 @@
   is_pull: true
   created_unix: 946684820
   updated_unix: 978307180
+
+-
+  id: 9
+  repo_id: 48
+  index: 1
+  poster_id: 11
+  name: pr1
+  content: a pull request
+  is_closed: false
+  is_pull: true
+  created_unix: 946684820
+  updated_unix: 978307180