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

routers/api/v1/api.go

@@ -571,5 +571,9 @@ func RegisterRoutes(m *macaron.Macaron) {
 				})
 			})
 		}, reqAdmin())
+
+		m.Group("/topics", func() {
+			m.Get("/search", repo.TopicSearch)
+		})
 	}, context.APIContexter())
 }