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

public/less/_repository.less

@@ -1733,3 +1733,12 @@ tbody.commit-list {
         }
     }
 }
+
+#topic_edit {
+    margin-top:5px;
+    display: none;
+}
+
+#repo-topic {
+    margin-top: 5px;
+}