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/repo.go

@@ -199,6 +199,7 @@ type Repository struct {
 	Size          int64              `xorm:"NOT NULL DEFAULT 0"`
 	IndexerStatus *RepoIndexerStatus `xorm:"-"`
 	IsFsckEnabled bool               `xorm:"NOT NULL DEFAULT true"`
+	Topics        []string           `xorm:"TEXT JSON"`
 
 	CreatedUnix util.TimeStamp `xorm:"INDEX created"`
 	UpdatedUnix util.TimeStamp `xorm:"INDEX updated"`