Add topic support (#3711)

* add topic models and unit tests

* fix comments

* fix comment

* add the UI to show or add topics for a repo

* show topics on repositories list

* fix test

* don't show manage topics link when no permission

* use green basic as topic label

* fix topic label color

* remove trace content

* remove debug function

models/fixtures/repo_topic.yml

@@ -0,0 +1,11 @@
+-
+  repo_id: 1
+  topic_id: 1
+
+-
+  repo_id: 1
+  topic_id: 2
+
+-
+  repo_id: 1
+  topic_id: 3