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

modules/auth/repo_form.go

@@ -516,3 +516,8 @@ type AddTimeManuallyForm struct {
 func (f *AddTimeManuallyForm) Validate(ctx *macaron.Context, errs binding.Errors) binding.Errors {
 	return validate(errs, ctx.Data, f, ctx.Locale)
 }
+
+// SaveTopicForm form for save topics for repository
+type SaveTopicForm struct {
+	Topics []string `binding:"topics;Required;"`
+}